27 CFR 26.68a: a Bond account.

Every person who files a bond under this subpart shall keep an account of the charges against and credits to the bond if the penal sum of his bond is less than the maximum prescribed in §§ 26.66(a), 26.67, or § 26.68, or if the penal sum allocated to his premises under § 26.66(b) is less than the prescribed maximum. He shall charge the bond with the amount of liability he accepts at the time he executes TTB Form 5110.51 or 5100.21, and shall credit the bond with the amount of the tax paid at the time he files each return, and remittance. The account shall also show the balance available under the bond at any one time.