37 C.F.R. 2.17(c)(2)

Once the applicant, registrant, or party has designated a practitioner(s) qualified to practice under §11.14 of this chapter, that practitioner may sign an associate power of attorney appointing another qualified practitioner(s) as an additional person(s) authorized to represent the applicant, registrant, or party. If the applicant, registrant, or party revokes the original power of attorney (§2.19(a)), the revocation discharges any associate power signed by the practitioner whose power has been revoked. If the practitioner who signed an associate power withdraws (§2.19(b)), the withdrawal discharges any associate power signed by the withdrawing practitioner upon acceptance of the request for withdrawal by the Office.

37 C.F.R. §2.193(e) Proper person to sign

Documents filed in connection with a trademark application or registration must be signed as specified in paragraphs (e)(1) through (9) of this section:

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1. (3) Powers of attorney and revocations of powers of attorney. Powers of attorney and revocations of powers of attorney must be signed by the individual applicant, registrant or party to a proceeding pending before the Office, or by someone with legal authority to bind the applicant, registrant, or party (e.g., a corporate officer or general partner of a partnership). In the case of joint applicants, registrants, or parties, all must sign. Once the applicant, registrant or party has designated a qualified practitioner(s), the named practitioner may sign an associate power of attorney appointing another qualified practitioner(s) as an additional person(s) authorized to prosecute the application or registration. If the applicant, registrant, or party revokes the original power of attorney, the revocation discharges any associate power signed by the practitioner whose power has been revoked. If the practitioner who signed an associate power withdraws, the withdrawal discharges any associate power signed by the withdrawing practitioner upon acceptance of the request for withdrawal by the Office.

Once the applicant or registrant has designated a qualified U.S. attorney, the attorney may sign an associate power of attorney, appointing another qualified attorney -- including one from a different law firm -- as an additional person authorized to represent the applicant or registrant. Note, however, that appointment of an associate attorney from a different law firm does not change the correspondence address from that of the attorney designated by the applicant or registrant. See 37 C.F.R. §2.18(a)(2); TMEP §§609, 609.01-609.02(b).

If the applicant or registrant revokes the original power of attorney, the revocation also discharges any associate power signed by the attorney whose power has been revoked. 37 C.F.R. §2.17(c)(2).

If the attorney who signed an associate power withdraws, the withdrawal discharges any associate power signed by the withdrawing attorney upon acceptance of the request for withdrawal by the USPTO. 37 C.F.R. §2.17(c)(2).