As I've been saying for years, you can type in any tag that makes sense to you. If people like it, it may catch on. I tend to use ADD for lyrics, and CHORDS or MELODY for specific chords or melody requests. Shorter tags leave space for more information. I use "Origins" for threads that have both background information and lyrics, and I try to consolidate the best information we have on a song into one "origins" thread per song.
I have always suspected that Max created the dropdown list of tags as the basis for some feature that never was fully realized. Only the "origins" and "permathread" tags have functions beyond being merely a text label. But the text labels do serve to standardize wording to make searching easier.
For a while, we had people thinking they had to start an "add" thread to submit an answer to a "request" thread. That got messy. Now, when I see that a request thread has an answer, I simply change the title to "add."
-Joe-