My idea for this site is that it should be comprehensible, and hopefully even useful, for people who don't speak the technical languages involved. In practice. this seems to mean insisting that each item here must be capable of producing usable output (either a picture of "the dots", or something you can hear). Thus, the site's idea of ABC is defined by the programs it uses to do that.
The way this is applied is that tunes are "validated" before entry into the main database. This is done by running the tune through one of these programs to make sure that it works (this is defined as "not crashing" plus producing some output; it may also generate error messages or warnings, which you will be shown). This is perhap more restrictive than it might be, but it means that there will be "something there" for all the tunes visible in the main listings, which should be of some use to people who don't want to have to read the ABC.
This checking will be done whenever you do a file upload or push the "Save" button in the editor. In the case of the file upload, any tunes that "fail" this test will automatically turn up in a separate "postponed" list, where you can sort them out at your leisure. In the case where the problem is caused by a subsequent unintended edit, there is an explicit 'Postpone' button that will do the same thing. if you'd rather not correct it instantly. Tunes placed in this list are not accessible by the normal means or visible in the tune listings, etc, they're withdrawn from the main database.