Just wrestled a user into knowledge of SELECTION SETS.

HERE's an easy UI idea; in the STATS panel--

here's a cheap fix: instead of saying "[none]" that damn line could say "[selection set]".

Right now, it's cryptic and 'undiscoverable' since it doesn't lead the user to click on it. "[selection set]" TELLS the user what the function of the line is.

When it's in POLY MODE it could be "[Part Set]".