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What Does the FLP in FLP_MainFeature stand for?

and any others for that matter.

I'd guess that FLP = "Feature Length Presentation"?

Some other common ones I see are "TMPL Splash" and "TMPL Loop", and I assume "TMPL" is short for "Template" and the "Splash" is the first screen of the disc menu background that's only played once, while "Loop" is then repeated (or looped) after the "Splash".

There's a heap of others, most of which are easy to guess ("SF" = "Special Features", etc) but the one to look out for is "EE" or "Easter Egg"!