The NFL announced in May that teams had voted in favor of allowing a 49th player to be dressed as an emergency third-string quarterback in regular and postseason games.
However, no specifics on the updated policy were released.
On Monday, the league broke down the new rule further ahead of a Week 1 matchup between the Kansas City Chiefs and Detroit Lions on Thursday.
Under the new rule, an emergency third-string quarterback must be on a team's 53-man roster and cannot be an elevated practice squad player, the league said in a Q&A on the updated policy.