NFL PUP List: Who's on It, and What Does It Mean for Each Player & Team? | News, Scores, Highlights, Stats, and Rumors

Teams can place players with football-related injuries on the Physically Unable to Perform list. There are two PUP lists; Active/PUP and Reserve/PUP.

Active/PUP is used in the preseason for players who suffered or are recovering from offseason injuries. As soon as their respective team's medical staff clears them, they're eligible to practice and play.

Reserve/PUP is used for the regular season. 

The deadline for a player to be placed on Reserve/PUP is August 31, which is the deadline for teams to get down to their 53-man rosters.

Most importantly: If put on the Reserve/PUP list, a player is not permitted to practice or play during the first six weeks of the season and is not counted toward the 53-man roster.

Here's an explanation regarding the window in which Reserve/PUP players can return the practice and subsequently return to the field that was featured in a July 2013 blog post of ESPN.com's Mike Sando:

Commencing on the day after the conclusion of the sixth regular season weekend (October 15) and continuing through the day after the conclusion of the 11th regular season weekend (November 19), clubs are permitted to begin practicing players on Reserve/Physically Unable to Perform for a period not to exceed 21 calendar days. Pads and helmets are permitted during the 21-day period. At any time during the 21-day practice period, or prior to 4:00 p.m., New York time, on the day after the conclusion of the 21-day period, clubs are permitted to restore such players on Reserve/Physically Unable to Perform to their Active/Inactive List.

Translation: After the sixth regular season weekend through the 11th regular season weekend is the time frame in which players on the PUP list can practice and rejoin their team. Once a player on the PUP starts practicing, they have a maximum of 21 days to take that player off the PUP list and onto either the active or inactive list. 

However, if a player practices during training camp or is taken off the Active/PUP list, he is no longer allowed to be designated to the Reserve/PUP list.