40 CFR 97.55: Banking.

NOX allowances may be banked for future use or transfer in a compliance account, an overdraft account, or a general account, as follows:

(a) Any NOX allowance that is held in a compliance account, an overdraft account, or a general account will remain in such account unless and until the NOX allowance is deducted or transferred under § 97.31, § 97.54, § 97.56, or subpart G or I of this part.

(b) The Administrator will designate, as a “banked” NOX allowance, any NOX allowance that remains in a compliance account, an overdraft account, or a general account after the Administrator has made all deductions for a given control period from the compliance account or overdraft account pursuant to § 97.54 (except deductions pursuant to § 97.54(d)(2)) and that was allocated for that control period or a control period in a prior year.