There's some pretty good links to US military specs on the Wikipedia page

https://en.wikipedia.org/wiki/Form,_fit_and_function

For example

https://apps.dtic.mil/sti/tr/pdf/AD0700066.pdf

I must admit that I try to avoid using those terms now, at least in a formal context. In critical systems it can become apparent that two systems are only truly FFF compatible if they are actually identical - which is not a useful definition! We all tend to get caught out in the end by systems that are intended to be FFF compatible but some niggly aspect of the detailed implementation means they aren't. So we might say at a project start "the intention of the project to develop B is that B will be FFF compatible with A", but we wouldn't formally say at the end "B is FFF compatible with A". What we'd say is "the following design parameters were determined for A, these have been replicated in B". I.e, those are the ones we identified, but we might have missed a crucial one! However, my job is to look at safety-critical systems, so that might well be over-pedantic for other equipment.

There's some pretty good links to US military specs on the Wikipedia page

https://en.wikipedia.org/wiki/Form,_fit_and_function

For example

https://apps.dtic.mil/sti/tr/pdf/AD0700066.pdf

I must admit that I try to avoid using those terms now, at least in a formal context. In critical systems it can become apparent that two systems are only truly FFF compatible if they are actually identical - which is not a useful definition! We all tend to get caught out in the end by systems that are intended to be FFF compatible but some niggly aspect of the detailed implementation means they aren't. So we might say at a project start "the intention of the project to develop B is that B will be FFF compatible with A", but we wouldn't formally say at the end "B is FFF compatible with A". What we'd say is "the following design parameters were determined for A, these have been replicated in B". I.e, those are the ones we identified, but we might have missed a crucial one! However, my job is to look at safety-critical systems, so that might well be over-pedantic for other equipment.