There’s a pretty detailed README file included with the project files. That part would be covered in the “Project Threepio’s file naming convention” section.

SW-eng-native.sup: A set of subtitles that cover what most people would want.
SW-eng-sdh-native.sup: A set of subtitles that include SDH (deaf & hard of hearing) audio cues.
SW-eng-alien-35mm.sup: Greedo’s subtitles, for use with preservations that don’t have burnt-in Greedo subs.
SW-eng-match-35mm.sup: A set of subtitles for the rest of the film, designed to look like Greedo’s subs (similar font & style).

SW means Star Wars, ESB means Empire, ROTJ means Jedi. I’d recommend against using SRT files unless you have no other option.

So, in short, -eng-native is probably what you’re looking for, unless you want SDH cues.