https://apnews.com/article/nfl-injured-reserve-pup-list-07369696d0a0c5a91f89ec9eea255a5d
Players put on the active/physically unable to perform list — or PUP — at the start of training camp have football-related injuries, like those on IR, but count against the active roster.
These players can participate in all team activities other than practice and can be activated at any point during camp when they are medically cleared.
A player can’t be placed on the PUP list after he has practiced once or played in a preseason game.
Players on the active/PUP list could be moved to the reserve/PUP list during roster cutdowns. If placed on the reserve/PUP list, a player wouldn’t count against the active roster and must sit out the first four games.
If a player is placed on the reserve/PUP list before final cutdowns, he will miss the season.