It appears the Lions have trimmed their roster down to 75 players. Earlier, Tyler Polumbus was let go, and the final move to get down to 75 involves cornerback Jack Williams, who was placed on the reserve physically unable to perform list.
Williams has spent all of training camp on the active PUP list, which is different than the reserve PUP list.
The active PUP list is for players who are injured at the start of training camp. Everybody on the active PUP list counts against the roster and can come off the list whenever. While on the list, though, players can't practice.
So what's the point of the active PUP list? Well, all players who end up on the reserve PUP list have to start out on the active one. If the end of the preseason is coming and the player is still too banged up to do anything, then he is moved from the active to the reserve PUP list, as the Lions have done with Williams.
What this means now is that Williams no longer counts against the roster and won't be eligible to play during the first six weeks of the season. By the seventh week of the season, the Lions will have to start thinking about whether or not they want to activate Williams or if they should just put him on injured reserve. If he gets healthy by then and the Lions have a need for another corner, then he could be activated come week seven. Until then, though, Williams is officially sidelined, which isn't too surprising considering he missed all of training camp.