Michael Jordan famously had in his contract with the Bulls a "for the love of the game" clause which entitled him to play in any game that he wanted to.
A lot of athletic contracts would forbid athletes from playing other sports or doing certain other strenuous activities - they were designed partially to try to avoid players injuring themselves doing something other than their contracted sport, and partially to prevent players earning money off their image outside of their team.
Jordan had a clause specifically exempting him from any restrictions around playing basketball - by his contract, he could play basketball in any league, or even any scrimmage, exhibition, or pickup game.
