Draft rights are a team's exclusive right to sign a player it drafted. Until the player signs or the rights lapse, no other team can sign them.
In HoopSim draft picks normally sign automatically on draft night. The exception is a team that drafts while its roster is already full: the pick is made, but the rookie is held as unsigned draft rights, off the roster and out of the free-agent pool, until the team makes room and signs them from the Roster tab on the rookie deal the pick carries. Rights that are still unsigned when the regular season starts lapse, and the player becomes a free agent. AI teams never hold rights; they make room with a cut instead. Players not taken in the draft become free agents at once.
Rulebook: Rookie deals. Related: rookie scale contract, draft lottery.