There’s a README file in the project which should explain this and other naming convention things.

“en-native” is what you want to use under most circumstances.
“en-template” is what to use if you’re making subtitles in another language from scratch
“en-full” is what you’d use in a preservation such as the Krieg der Sterne project, where the crawl has been translated into another language, and the alien subtitles have either been translated or removed

Also, generally speaking, don’t use the SRT files unless the SUP files won’t work for your purposes.