FWIW, YS only if needed is still ASSN (assigned), but it shows which preference was used in the assignment.

I think one thing many don't understand about YS on Short Call is that there is an order to the assignments/awards. The order is actually pre-posted on Deltanet. Basically, certain time slots are in a certain order, and it cycles through those time slots. YS preferences are (presumably) considered in that order.

For example, if you have a YS in for "only if needed for SC" for the 1800 slot, all of the 1800 slots might be filled (in order) before you are "needed", and you end up with the 1000 slot.

Meanwhile, "first in sequence for SC" YS should get you the first crack at a SC time slot in the order, at the risk of being AWARDED SC when you might otherwise have not been assigned SC.