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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)?

2TSFIs should be expanded to either TOE Security Function Interfaces or Target of Evaluation Security Function Interfaces. It's almost always the case for a long noun phrase that the main noun (also known as the head word) is the last word in the phrase. – Damkerng T. – 2014-09-14T12:50:37.390

5It is very common for the head noun to be the final word in a noun phrase, but "almost always" is too strong. Many nouns take complements as postmodifiers (RFCs = Requests for Comment), and there are a variety of other postmodifiers, for example relative clauses, which can follow the head noun. – snailplane – 2014-09-14T13:00:07.013

1Technically, RFCs = Requests for Comments (with an 's' at the end), but it's too late to edit my old comment and that makes the example less suitable than I'd like. Here's a less ambiguous example: AGs = Attorneys General. – snailplane – 2014-09-14T13:21:22.973

1There are also some abbreviations where you can't pluralize the expanded form by pluralizing to one of the words. For example, people use POWs and MIAs. This would probably be expanded to "prisoners of war and servicemen missing in action". You certainly wouldn't use "missing in actions" or "missings in action". – Peter Shor – 2014-09-14T14:08:23.547