Steve,
The basic rule for this league is that you can use any player that ever played for your franchise so long as he had one AB or one IP. Each season one manager gets to create a rule along this theme, and that rule may limit the players available to you. The idea is that you get a chance to make a rule that helps your franchise and might even improve the competitive balance between the haves (Yankees, Giants...) and the havenots (Browns/Orioles, Senators/Twins...).
This time around, every position player has to have a 1 rating defensively. It's a three step draft. First you get to "protect" 10 players carded to your franchise. Those players are off limits to everyone else until you release them after the draft. As Dave pointed out, someone like Mays isn't available to anyone else, so you have no need to protect him.
Then we have a 6 round live draft in which you can select any player that ever played for your franchise as long as they meet the AB / IP requirements. The remaining players are picked up in the Autodraft, but still must have played for your franchise at some point in their career.