What do you mean "your pick is out"

Does that mean the team that I picked is no longer a choice I can make from the pool for the remainder of the season?

Or

Does that mean I have one less choice each week for the remainder of the season?

Or

Does it mean something else?

Also "number of picks you buy determines how many choices" - I assume one pick=one choice?

Also, 13 teams? Since that's not all of them. Do you only pick between teams in the subset of games that are between those 13, or between all games or between games where they don't play each other?

If you can pick from games they play each other, are you allowed to pick both sides (assuming you have 2 picks left?)