47 C.F.R. § 1.8003
The FRN must be provided with any filings requiring the payment of statutory charges under subpart G of this part, anyone applying for a license (whether or not a fee is required), including someone who is exempt from paying statutory charges under subpart G of this part, anyone participating in a spectrum auction, making up-front payments or deposits in a spectrum auction, anyone making a payment on an auction loan, anyone making a contribution to the Universal Service Fund, any applicant or service provider participating in the Schools and Libraries Universal Service Support Program, and anyone paying a forfeiture or other payment. A list of applications and other instances where the FRN is required will be posted on our Internet site and linked to the CORES page.
47 C.F.R. §1.8003