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A lot of these articles use abstruse acronyms. Is there perhaps a place where they're all compiled with their explanations?

For instance, what is (U)? Or (S), (SI), and (REL)?

U : Unclassified, S : Secret, SI : special intelligence (an SCI control marking), REL : marking that designates release

There's a bunch more too:

Yes, such as "EPICFAIL".