47 C.F.R. § 1.8001 – FCC Registration Number (FRN).

(a) The FCC Registration Number (FRN) is a 10-digit unique identifying number that is assigned to entities doing business with the Commission.
(b) The FRN is obtained through the Commission Registration System (CORES) over the Internet at the CORES link at www.fcc.gov or by filing FCC Form 160.