Assume that TSFI stands for TOE Security Function Interface. And TOE (The first word) stands for Target Of Evaluation.
I want to know in the TSFIs the plural
s returns to which word(s)?
The rule in expanding acronyms for pluralization, or in pluralizing any noun phrase, is to first figure out what the noun is. It's not always the last word!
In the "TSFIs" example, everything up to "interfaces" is a modifier of what kind of interface it is, therefore "interface" is the noun to be pluralized.
In the "RFCs" example "for Comments" is a modifier of "Request", so "request" is the noun to be pluralized.
In the "Attorneys General" example, "attorney" is the noun being modified by "general", so it's the noun to be pluralized.