Larry in TN
En-Route
It's not 91 or 121. It's straight out of 14 CFR 61.51.Call it 91 vs 121, single crew vs two crew, ATP vs PP, or whatever. I may be using the wrong verbiage on that point.
61.51 (e)(2) If rated to act as pilot in command of the aircraft, an airline transport pilot may log all flight time while acting as pilot in command of an operation requiring an airline transport pilot certificate.
and
61.51 (f)(1) through (3) for SIC.