In major American sports, rookies are required to join the official league draft to enter professional teams. This draft is typically characterized by the following:
Rookies do not get to choose which team they join
Rookies are given a set salary based on when they are chosen in the draft (e.g. 1st round pick, 2nd round pick)
A free agency model allows athletes to negotiate their contracts directly with prospective teams. They are able to negotiate their salaries, length of stay, and other terms.