There should be a README file included which explains it all. In general:

full: everything is subtitled
titles: only onscreen text is subtitled, to accompany a dub that does not voice over the onscreen text (and/or alien languages)
native: no onscreen text is subtitled, designed to accompany video that already includes burnt-in alien subtitles
nocrawl: the opening titles and crawl are not subtitled, typically designed to accompany video that does not include burnt-in alien subtitles
match: same as native, but designed to mimic the appearance of theatrical alien subtitles
alien: subtitles for alien dialogue only, designed to look almost exactly like the theatrical alien subtitles
template: used only for creating new subtitles
compat: indicates that the text is specially formatted for broader compatibility with media players

English also has “mono”, which is for use only with the mono mixes, which have slightly different lines and timing from the more common stereo/6-channel mixes. At first I thought there was more than one mono mix for Empire, but there’s only one so the 16mm nomenclature has been removed in the most recent versions and it’s just “mono” now.

If you want the English subs for the default English audio, what you want is -native, or -sdh-native, depending if you want the other audio cues for the hearing impaired or not.

Also, ideally, you wouldn’t be messing around with SRT files at all. I think I lost this fight long ago, but the complete Project Threepio files include SUP files, which are better all-around and don’t some in quite so many different flavors. But they’re considerably larger, so are not included with Despecialized.