The difference between a goal and a subplot is typically defined as follows. A goal is a personal objective. A subplot is a portion of a story that is aside from the main plot.

Subplots don't have much to do with objectives, but are instead a way for players to contribute their own plot concepts.

This should be asserted more strongly by both game explanation and examples, perhaps even changing the way they're played.

I think a subplot should permit player created challenges.