Captain Ned wrote:
SP = Starting Pitcher
RP = Relief Pitcher
You can put either in a slot marked just "P".
You want some of both, as SPs get you wins and RPs get you saves.
Yes, usually you have more SPs than there are SP slots and (usually) more RPs (closers) than you have RP slots. Since SPs only start once per five days, you rotate them into the SP slots as their starts come up. On the odd day when you have more than two SPs starting, you stick the extras in the P slots. The rest of the time, you want to have the closers sitting in the P slots because you never know when they might pick up a save. Note that this requires you to adjust your roster regularly to rotate guys into and out of these spots, because inevitably you'll have some sitting on the bench and you don't want them there on days when they get their start. However, because rotations are generally on a fairly rigid 5-day schedule you can usually set your rosters for every day of the entire week on Sunday evening, and then just check in periodically to adjust things in case of injury, rain-outs, starts getting pushed back, etc. You have to do that for the position players anyway, because Mondays and Thurdays are often off days where you have to juggle guys off your bench if you're trying to maximize your counting stats. Note that Yahoo tends to carry forward a roster from one day to the next -- eg, if you swap a guy into the SS position from the bench for Monday, he'll be there on Tuesday as well. That's annoying if you only put him there because your regular SS had no game on Monday. So what I do is roll ahead to Tuesday, set my lineup there (maybe just swap a starter around from SP to P or whatever) and then go back and do the major one-off adjustments that Monday requires, without having it affect later days. (And ditto for Friday and Thursday).
Some folks who are serious about fantasy are constantly (ie daily) adding and dropping SPs, so that they have a starter or two going every day -- eg you pick up Lance Lynn for his start yesterday, and then drop him and replace him for Erasmo Ramirez who is starting today. And on and on. In the fantasy world this is called "streaming" -- and that's way too much work for me, especially for a casual league where no money is at stake. Some folks seem to enjoy it (or can't help themselves) but I also find it can bite you in the ass, because the guys you're picking up are by definition not good enough to keep so they frequently have bad starts, and while it means you might accumulate extra Ks or Ws, a couple of bad starters can blow up your K/9 and BB/9 stats.
A less extreme version of this for people who don't want to be that committed to daily roster fiddling is to look for 2-start pitchers: the guys who are getting starts on Monday and Tuesday will (usually) get another on Saturday and Sunday (respectively), so if they're decent pitchers they might be worth an add for the week, to be replaced by another 2-start pitcher the following week. This fits nicely if you're in the habit of doing all your roster adjustments on Sunday night.