In the absence of instructions by the taxpayer to the contrary, and unless otherwise provided by law or by regulation, tax credits shall be applied to the penalties, legal costs, interest and tax principal, in that order, of each tax debit in the taxpayer's account, in the chronological order in which the tax debits arose.
61 Pa. Code § 151.24
This section cited in 61 Pa. Code § 151.21 (relating to definitions); and 61 Pa. Code § 151.22 (relating to transfer, assignment and refund of credits).