A max contract pays a player the most the league allows, a percentage of the salary cap that rises with their years of service. A "supermax" is a real-league variant that lets a team pay a qualifying star an even higher percentage.
In HoopSim the maximum salary is 25% of the cap for a player with 0 to 6 years in the league, 30% with 7 to 9 and 35% with 10 or more, by default. There is no separate supermax; the 35% tier is the top. Raises run 8% a year when you re-sign or extend your own player and 5% when they sign with a new team, and no year can rise above their max. Full Bird rights let you pay the max over the cap but never past the hard cap.
Rulebook: Re-signing. Related: Bird rights, contract extension.