An apron is a payroll line above the luxury tax that takes team-building tools away from the teams that cross it. Pro basketball's rules have two: the first apron tightens trades, and the second apron, higher still, locks a team out of most ways to add salary.
HoopSim has both. The first apron is $195.9M by default: above it you can take back at most 110% of the salary you send out plus $100K in a trade, and you cannot combine contracts to make the match. The second apron is $207.8M by default and works as a hard cap: no trade, signing, exception or Bird-rights deal can take you past it, you can never take back more than you send plus $100K, and the low-level exception is off. Minimum deals still go through while you have fewer than 15 players. The commissioner can move either line.
Rulebook: Aprons. Related: hard cap, salary matching.