Part 15, Section 31, Paragraph 6:
(6) Digital devices authorized by verification, Declaration of Conformity, or for which an application for equipment authorization is filed on or after May 1, 1994, and intentional and other unintentional radiators for which verification is obtained, or for which an application for equipment authorization is filed on or after June 1, 1995 are to be measured for compliance using the following procedure excluding section 5.7, section 9 and section 14: American National Standards Institute (ANSI) C63.4–1992, entitled ‘‘Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 9 kHz to 40 GHz,’’ published by the Institute of Electrical and Electronic Engineers, Inc. on July 17, 1992 as document number SH15180. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. The Commission encourages the use of this procedure for testing digital devices, intentional radiators, and other unintentional radiators as soon as practical.
My verbiage might be a bit dated as part 15 received a major rewrite/update in 2002 (shown above) and it has been somebody else's job since then. I only inspect boards here and there for glaring design blunders that will lead (or have led) to excess conducted or radiated emissions. My favorite is supply tracks around a board's perimeter that resemble (and behave like) UHF loop antennas.
If you've stayed within the conditions necessary for the transmitter and receiver to maintain their FCC certification, you don't need to formally register your product with the FCC, you can simply declare its compliance with a decal saying it has been found to comply with FCC Part 15 yada yada after it passes testing.
It also refers to testing as soon as practical. Different manufacturers and different customers interpret this different ways. We have a few that insist on testing before accepting delivery. Most will accept evidence that similar products have been found to be in compliance and having something from every major project category tested at some point.