"Multi" is the default icon that gets added to a fic if you check two or more of the other categories. So a Harry/Draco-and-Ron/Hermione fic tagged "M/M" and "M/F" will get that multicolored icon when it appears in search results.

(Although it does not appear to be added as a tag -- I just tried to search an M/M+M/F fic of mine, and it didn't come up under the filter.)

Either way, I always thought "more than one of these categories" was what the tag was designed for, rather than referring only to poly relationships. So it doesn't seem like the same kind of tag abuse that can be complained about with F/F.