There was some confusion about the PUP (Physically Unable to Perform) lists at 18 to 88, so I did some research. What is the PUP list? What's it's purpose? Why would a team put a player on it?
The purpose of the PUP list is to allow teams to keep players that come into the season injured, but are expected to return during the season without having to carry them on the very limited 53 man roster. A player on the PUP list can't practice with the team, but can rehab and attend meetings with the team.
For training camp there are two PUP lists, active and reserve. The reserve PUP is the one you hear about during the regular season. The player can't return until after week 6. After week 6 the team has 3 weeks to IR, release, or add the player to the roster. Players on the reserve PUP list count against the roster limit until the rosters are cut to the final 53, then they don't count for up to 9 weeks of the season.
The active PUP list is for training camp. Players can't practice and can't be put on the list once they have practiced. Players can be taken off the list at any time. Players on the list count against the 80 man camp limit.
So why use the active PUP list?
A player must be on the active PUP list to be able to be put on the reserve PUP list.
So the players are being put on the active PUP list to keep the option of the reserve PUP list open. Any player that can't practice at the openning of camp where there is any chance the injury could not be recovered by September 1 (when the 53 man limit applies) they will be put on the list so that the reserve PUP list is an option for them.
Peyton Manning, Bob Sanders, Dwight Freeney, Ryan Lilja, Tyjuan Hagler and Tom Santi are on the active PUP list.