An unrestricted free agent is a player whose contract has ended and who can sign with any team, with no right for their old team to match.
In HoopSim every player whose deal runs out is unrestricted unless their team issues a qualifying offer, which is only possible for young players and first-round rookies. Unrestricted free agents go into the free-agent pool when the offseason starts and are bid on in three sealed-bid rounds (July 1 to 10, 11 to 20 and 21 to 31 on the league calendar by default), then a free-for-all through August. Players weigh money, contract length, winning, role and loyalty, and each has a read on your offer (Likely to accept, On the fence, Likely to pass) drawn from your scouts' view of them. Their old team keeps its Bird rights while they sit in the pool, so it can still outbid others over the cap.
Rulebook: Making offers. Related: restricted free agent, Bird rights.