BPAs are covered in FAR part 13. A BPA is an agreement between the parties, therefore a BPA is not funded. An agreement is not a contract, however, each order placed against the BPA is a contract and must be funded by the organization.

NAF funds are covered in the AFMAN located at https://static.e-publishing.af.mil/production/1/saf_aq/publication/afman64-302/afman64-302.pdf

Chapter 10 speaks to Blanket Purchase Agreements specifically. No prohibition exists in the FAR, DFARS, or DFARS PGI that puts a limitation on the funds available for use.

If an AFI exists, it did not surface during my research.