It applies to any ops.mlf,
In looking a the Carpenter letter, I assume it refers to Part 121 ops.
Where it mentions maintaining instrument currency, I don't recall ever keeping track of/documenting that the way we had to for landings, for example. I'm sure the company would've had us submitting some kind of form if it had been required.
I have almost no Part 91 operating experience so was not exposed to having to keep track of instrument currency. Should I have been keeping track when I shot an approach in actual IMC under Part 121 ?
Too late now but just curious.
chances are, your Part 121 checks probably satisfy Part 61 currency standards.