INTRODUCTION
T1 The Union Oil Company of California (Unocal) petitioned this court for review of a Utah State Tax Commission (Tax Commission) decision holding that Unocal was not entitled to a refund of severance tax payments. We affirm the decision of the Tax Commission in part and reverse in part.
PROCEDURAL BACKGROUND
4 2 In 1997, Unocal filed for a refund of the severance taxes it paid for the years 1994 through 1997. The Commission paid the refund, then sent it to the Auditing Divigion to be audited. The Auditing Division determined that the original tax filings were correct, and that the refund had been provided in error. Consequently, in 2000 the Auditing Division sent Unocal a deficiency notice for the 1994-1997 period (First Assessment Period). Unocal petitioned for a redetermina-~ tion, which the Tax Commission stayed pending the resolution of a severance tax appeal by ExxonMobil, which was then pending before us. Unocal filed an amicus brief in the ExxonMobil matter. We issued our opinion in November of 2003 in favor of ExxonMobil. In the opinion we expressly limited any retroactive application to ExxonMobil. ExxonMobil Corp. v. Utah State Tax Comm'n,
1 3 In 2004, the Auditing Division issued a second severance tax deficiency notice to Unocal, along with a conservation fee defi-clency notice, for the years of 1998 and 1999 (Second Assessment Period). Unocal appealed the new deficiency assessment to the Tax Commission. The Tax Commission consolidated the appeal with Unocal's first pending appeal and held a formal hearing in 2007. The Tax Commission upheld the deficiency assessments against Unocal in a final decision issued on December 24, 2007. Unocal then petitioned us for review, which we granted.
FACTUAL BACKGROUND
4 During the period of January 1994 to June 1999, Unocal was the designated Unit Operator for oil and gas production from the Lisbon Unit, located in San Juan County, Utah. The Lisbon Unit is comprised of federal and state oil and gas leases. Unocal operated multiple wells within the Lisbon Unit and operated a processing facility, the Lisbon Plant, just outside the legal parameters of the Unit. The oil and gas extracted from within the Lisbon Unit was "sour," meaning that it included various contaminants, including several natural gas liquids (NGLs) that could be separated out and sold. The audits at issue in this review involve the production of oil, gas, and NGLs from the Lisbon Unit.
1 5 Unocal was required by state statute to pay severance taxes on the value of the oil and gas it extracted under its lease agreements. See Utah Code Ann. §§ 59-5-101 to -215 (2000). 1 The statute provided the details for valuing the oil and gas for taxation purposes, including allowing for several valuation methods. Id. § 59-5-102. For the First Assessment Period, Unocal valued the oil and gas at the point of sale in its original severance tax calculations, but later filed for a refund based on calculations using the net-back valuation method. Though the Tax Commission initially paid the refund, the Auditing Division determined that the refund was in error and issued a deficiency notice using different valuation methods-the contract prices at the Lisbon Plant tailgate for the oil and gas, and the net-back method for the NGLs. The NGL calculation included a processing costs allowance of two-thirds. For the Second Assessment Period, Unocal used the net-back method of valuation. The Auditing Division issued a deficiency notice, *1161 relying on the posted prices for "sweet" oil produced in a different field and using the tailgate sale price for the gas and NGLs.
T6 In addition, Unocal was eligible for a statutory annual tax exemption on the combined taxable value of its oil, gas, and NGLs. The Auditing Division applied the full tax exemption in 1998 and prorated the exemption at fifty percent in 1999 because Unocal was the Lisbon Unit operator only until June 1999.
T7 Unocal challenges the Auditing Division's tax valuation methods and the pro-ration of the annual exemption.
STANDARD OF REVIEW
T8 We review the Tax Commission's interpretation of case law and statutes for correctness, according the Tax Commission no deference. See ExxonMobil Corp. v. Utah State Tax Comm'n,
ANALYSIS
I. THE PROSPECTIVE RELIEF DECLARATION OF EXXONMOBIL
I 9 We first address Unocal's request that we overturn our express limitation in Exzon-Mobil and allow retroactive application of its holding to this case.
1 10 While Unocal's appeal before the Tax Commission was pending, we decided ExxonMobil Corp. v. Utah State Tax Commission,
{11 Following ExzonMobil, Unocal's appeal recommenced before the Tax Commission, and Unocal sought to have the Exzon-Mobil holding applied to its severance tax assessments. The Tax Commission declined 'to do so because Unocal's appeal was pending when ExxonMobil was decided and was therefore outside the scope of the limited holding. Unocal asks us to overrule the limitation on retroactivity in order to apply the ExxonMobil holding to its appeal.
112 The Tax Commission opposes our consideration of Unocal's request, in part on the theory of res judicata. But we need not wrestle with the application of res judica-ta. As the court of last resort on matters of state law, we are always at liberty to review, and revise, our prior decisions. While legal principles like res judicata play an important role in the predictability and stability of our legal framework, they always bow to the wisdom of time when a prior action or decision is truly in need of change.
*1162 13 We are convinced that our decision in ExszonMobil requires some modification and clarification. The concern for upheaval caused by requiring governmental units to repay already committed and spent funds is still a valid one, and our express declaration that the holding in ExxonMobil is to be limited to prospective matters still applies in part. However, as explained hereafter, we address questions of application differently.
II, APPLICATION OF EXXONMOBIL TO PENDING PROCEEDINGS
Unocal alternatively asks us to clarify the seope of ExxonMobil's retroactivity limitation. This task requires that we look first to the plain language used by this court to determine the seope. See Kennecott Corp. v. Utah State Tax Comm'n,
15 In ExxonMobil we stated, "whether in refund requests or deficiency proceedings, as to all but ExxonMobil the rule announced today is to have prospective application only." ExxonMobil Corp. v. Utah State Tax Comm'n,
116 The Tax Commission interprets our holding, and the word "pending," to refer to the period being taxed, rather than the time at which proceedings are commenced before the Commission. Thus, proceedings based on taxes accrued during periods prior to ExzonMobil would be immune from application of its holding, regardless of when they were initiated. The Tax Commission cites American Trucking Ass'n, Inc. v. Smith,
117 Accordingly, because the Tax Commission issued the deficiency notices against Unocal for the Second Assessment Period after the announcement of ExzonMobil, they were commenced after ExxonMobil and were not "pending." Therefore they are not subject to analysis under the rule announced in ExxonMobil.
£18 The Tax Commission contends that because the Second Assessment Period deficiency appeal was consolidated with the deficiency proceeding for the First Assessment Period, which was pending at the issuance of ExxzonMobil, the Second Assessment Period deficiency appeal should also be considered part of the "pending proceedings." This argument is unpersuasive. The fact that the proceedings were consolidated only confirms that they were separate matters at the outset, one of which was commenced before ExxonMobil and one afterwards. We are *1163 unwilling to apply a relation back analysis to such a situation. We thus hold that the deficiency proceeding against Unocal for the Second Assessment Period was not pending at the issuance of ExzonMobil and is subject to the holding announced therein.
19 Conversely, the deficiency proceeding against Unocal for the First Assessment Period was pending at the issuance of Exxon-Mobil and clearly falls within the limitation on retroactivity. However, deficiency assessments do not implicate the same revenue concerns as refund requests. As we explained, albeit briefly, in ExxonMobil, our limitation on retroactivity was to "protect the solvency of governmental entities" from the potential burden of "large refunds of taxes already collected and spent."
120 Given this distinction and the implications for the public treasury, we are persuaded to refine our limitation on retroactivity with respect to deficiency assessment matters pending at the time of our decision in ExxonMobil. Deficiency assessments issued by the Tax Commission but unresolved at the time of our decision in ExxonMobil fall within the ambit of the ExxonMobil analysis. As before, the ExxonMobil holding will not apply retroactively to those pending matters that are taxpayer initiated refund requests. Consequently, the deficiency assessments pending against Unocal for the First Assessment Period are subject to ExxonMobil.
III. REMAINING VALUATION ISSUES
121 Finally, Unocal appeals a number of valuation determinations in the Tax Commission's decision. These determinations were made without application of Ezx-onMobil's point of valuation guidance. Because we hold that both of Unoecal's appeals are subject to the valuation definition set forth in ExzonMobil, we remand the valuation issue stemming from the point of valuation to the Tax Commission for consideration under the analysis and rule as set forth in ExxonMobil. We affirm the Tax Commission as to the remaining valuation questions.
122 We discuss only Unocal's appeal of the application of the annual exemption for guidance to the Tax Commission and parties generally. Utah Code section 59-5-102(2)(a) exempts "the first $50,000 annually in gross value of each well or wells" from the severance tax and provides that the exemption is "to be prorated among the owners in proportion to their respective interests." Utah Code Ann. § 59-5-102(2)(a) (2000). In accordance with the statute, the Commission gave Unocal a single exemption for all wells located within the Lisbon Field for each year through 1998. In 1999, because Unocal sold its interest in the Lisbon Field halfway through the year, the Commission reduced the annual exemption by fifty percent.
123 Unocal contends that this application of the exemption was error because under the statutory language it is entitled to the full exemption for each individual well within the Lisbon Field and should receive the full exemption for 1999 where the first $50,000 in gross value was produced while Unocal retained its interest in the production.
{24 In considering Unocal's contentions, we look to the statute's plain language. Evans v. State,
$25 The statute goes on to provide that the exemption "be prorated among the owners in proportion to their respective interests in the production or in the proceeds of the production" of "the first $50,000 annually in gross value." Id. § 59-5-102(2)(a). Unocal contends that the legislature intended that the exemption be prorated among the parties who share ownership when "the first $50,000" of oil or gas is extracted from the wells, rather than among the parties who share ownership of the entire annual production. Were this to be the case, a party who had an ownership interest in oil and gas production only during the latter part of a year, as here, would not be entitled to any portion of the exemption, though it bore its share of the costs and burdens of production. We believe such an interpretation runs contrary to the accepted principle that "taxes should be imposed equally and without discrimination so that everyone bears his fair share of the tax burden." See Parson Asphalt Prods., Inc. v. Utah State Tax Comm'n,
126 The other valuation issues presented by Unocal are within the discretion of the Tax Commission to determine. As we find no error in the Tax Commission's determinations, we remand only the question arising from the point at which the oil and gas are to be valued for consideration of the appropriate valuation as set forth in ExxonMobil.
{27 We note that the centerpiece of the ExxonMobil controversy has been the circumstances under which it is subject to retroactive application. Our resolution of the controversy here does not actually concern prospective application. Rather, after Union Oil, a claim's status as "pending" will be inconsequential for severance tax calculation purposes. Instead, the nature of the claim will determine whether it survives ExszonMo-bil. In every deficiency action, the Exxon-Mobil valuation formula will apply. Where refunds are sought, however, ExxonMobil will have only prospective application, except as to ExxonMobil itself, We acknowledge that this development represents a substantial modification of ExxonMobil.
1 28 Affirmed in part, reversed in part, and remanded for further proceedings consistent with this opinion.
Notes
. These sections have since been amended. We cite to the version in effect during the relevant time period.
. See Parson Asphalt Prods., Inc. v. Utah State Tax Comm'n,
