I always considered "junk" to be a simple-named category for miscellaneous items not fitting anywhere else.

That would make sense. Just checked in the map editor, and items listed under "Junk" make it really look more like a "Miscellaneous" category.

Btw, I just set a skill ticket on fire. Unlike music tickets, which one can find by the hundreds, skill tickets are not marked as precious. Hmm.