The Jets have placed Muhammad Wilkerson, Devin Smith, Khiry Robinson, James Carpenter, Breno Giacomini, and Kendall James have been placed on the active/physically unable to perform (PUP) list, the team announced.
This usually means that the player is recovering from an injury and not yet ready to practice. However, it can also mean that they failed their conditioning test, in which case it's not uncommon for the player to retake the test the following day and then be removed from the list.
In most cases, this is just a procedural move protecting the team's right to retain them on the PUP list in the event they suffer an injury or setback before they are removed from the list and attend practice.
Once you have attended a practice, there is no mechanism by which you can be placed onto (or back onto) the PUP list, although temporary injured reserve remains an alternative. Players on the PUP list can still attend meetings and work with team trainers.
It's important to note that anyone on the PUP list still counts toward the 90-man roster limit, so the Jets will not be able to make any further signings in lieu of activating these players unless they make corresponding cuts.
However, once the season begins, if you are still on the PUP list, you don't count toward the roster limit and can be activated at any time between week six and week 10. That's what happened with Stevan Ridley last season.