A player is usually added if it looks like they may get taken in the Rule 5 draft, five years of play after age 18 or four years if signed after age 19. They don't need to be added. But can be added after their 6th year in the minors, when they would be eligible to become a free agent and sign with anyone they please. Once on the 40-man, they can be optioned out to the minors for three seasons, before having to stick with the team. Then it becomes arbitration, free agency and the ilk. If a player on the 40-man is actually removed from the 40-man, he can be claimed by another team that has to place them on their 40-man, and keep the cycle going if they wish to release the player. If he isn't claimed by anyone, he can go back to the team's minor league. So, if you sign a player at 18, h has five years before he needs to be added to the roster. Then you can control his up/down for four more years while they gather service time and all. But every year many players become 5-year free agents (used to be 5). Twins signed a whole mess of them, plus got a couple that were on 40-man rosters (Gray, Maloney, Mastro) that they have to keep, but could lose. Sometimes players come off injuries, which was why the Twins were able to send out Slama and Bromberg last fall...no one claimed them. A couple of years back, the Twins demoted Glen Perkins. Then they removed him from the 40-man and NO ONE claimed him. But he made it back to the roster. If you make the 49-man, you pretty much WILL PLAY ion the majors. Exceptions...always that A or AA guy that is protected from the Rule 5 than never really pans out (de los Santos a few years ago, Arcia maybe this year). When players are added to the 40-man in the Fall, they have to stay on thru the end of spring training...you can't protect them, then send them right back to the minors. You can if you add them in September or during the season. A few players stay on the 40-man thru the winter meetings (like a Butera, for example) so you have an option to replace them if someone becomes available.