I was not part of TOSEC at that time (of the music dats) and don't know many details about them. Maybe the unknown was just a dat full of music sets not yet renamed (similar to what happens with Yoshi's dats?).
As for the "seq" flag, i have no idea, you are talking about more info flags, right? Currently on TOSEC dats there is no set using it, i guess it is something related with the music files or something that is no longer used, maybe Cassiel or other can give you an hint.