When I look at my Music 1 LE categories, some songs have a “P” next to them. What does that represent?.

This shows in the LE version only.  P means: Played.    As M1 begins scheduling from the top of the category card stack. As a song is scheduled, it gets this visual marker within its category, the P. When the last song in the category is scheduled, it clears all the P's and begins at the top of the stack again. If you scroll down a category and see the first songs with no P and then open a new schedule and click to select the category on a clock then, in the search depth/selections list at the bottom of the window, those no-P songs will the the ones which appear.M1 will not necessarily schedule the songs in 1-2-3-4 rank order. As the rules come into play during scheduling, it might drop in the songs in an order like: 2-1-4-3 or some such. But when the category is run-through and the played flags are cleared, the songs will retain their original ranking. So, it is not operating like a 'true' card stack, pull from the front, place to the rear. We tried working it that way early on and found the 'hard to play' songs (those with numerous rules attached to them) tended to bunch-up, giving more and more scheduling problems as you'd eventually get to a point where nothing but hard-to-play songs were available at the top of the card stack.