You got it steve. Via the Set up on page 1:
* Hitters & Starting Pitchers can begin their League careers in either their first $5 million+ salaried season, or an earlier cheaper season if you prefer. (Also aplicable to Hitters/Starting Pitchers that don't have a $5 million Season. In these cases, you may choose any season to begin with, i.e.- Oscar Gamble.)
* Relief Pitchers can begin their League careers in their first $3 million+ season, or begin at any earlier cheaper season beforehand.
* This is to ensure that we all have a team full of usable players instead of teams full of Rookies that have 200 PA's and hit .215 for 3 years... You can always opt for a lower season. A lot of great reliever seasons are less than $3 mil, and many of the great Part-time players and pinch hitters will be less than those benchmarks as well.
* The season you choose as owner will be the season that player will be officially “Born into the League”.