N.Y. Comp. Codes R. & Regs. tit. 20, § 8-1.3 – Assessment of tax on combined reports.

(a) Where the tax is computed on the basis of a combined report, the commissioner may assess the entire amount of the tax and all additional taxes computed on the basis of such report against any one or more of the taxpayers covered by the combined report, in such proportions as the commissioner determines, but every such taxpayer is liable for the entire amount.
(b) In the case of a taxpayer which computes its tax on the basis of a combined report where the requirements for filing a combined report are not met or in the case of the inclusion of one or more corporations where the requirements for inclusion of such corporation or corporations are not met, the tax assessed is, in the case of corporations:
1. (1) which do not meet the requirements to file on a combined basis, the amount required to be shown on the separate reports required to be filed pursuant to section 6-2.1(b) of this Title, or would have been shown if the taxpayer had filed its report separately; and
2. (2) which meet the requirements to file on a combined basis, the amount that would have been required to be shown if the reports had been filed in combination only with those corporations which meet the requirements for filing on a combined basis.