AFAIK, you are probably not going to find the answer in the FARs. Any requirements for "must use" would be covered under the Ops, M or T-specs, or LOA, associated with the particular operation. I.e., there is no one, overarching answer. The answer will vary from op to op.
These references are from 2013, not sure there are any newer, but they do call out the relevant FARs and other guidance:
https://www.faa.gov/documentLibrary/...N_8900.237.pdf
https://www.faasafety.gov/files/gsla...5375/NVGS2.pdf