See paragraph (a)(2)(ii) of § 20.6161-1 for the meaning of the term "undue hardship". An example of undue hardship is a case where, by reason of the time required to settle the complex issues involved in a trust, the decedent's heirs or beneficiaries cannot reasonably expect to receive the decedent's remainder interest in the trust before the expiration of the period of postponement. The extension will be granted only in the manner provided in paragraph (b) of § 20.6161-1 , and the amount of the tax for which the extension is granted, with the additions thereto, shall be paid on or before the expiration of the period of extension without the necessity of notice and demand from the district director.
26 C.F.R. §20.6163-1