I've noticed A LOT of confusion about the practice squad and Physically unable to perform rules so I thought I'd do a quick crash course as of how I understand the rules, at least a very simplified version of this.
I'll do the PUP list first because it's really easy to understand.
Physically Unable to Perform list
What you need to know about the Physically Unable to Perform list is this--If the Player has practiced EVEN ONCE this year he is NOT eligible for the Physically unable to perform list.
If a player is not Already on the Physically unable to perform list they can not go there this season.
No onto the Practice squad rules.i
Directly from the Collective Bargaining Agreement
Section 4. Eligibility
(a) The Practice Squad shall consist of the following players, provided that they have not served more than two previous seasons on a Practice Squad: (i) players who do not have an Accrued Season of NFL experience; and (ii) free agent players who were on the Active List for fewer than nine regular season games during their only Accrued Season(s). An otherwise eligible player may be a Practice Squad player for a third season only if the Club by which he is employed that season has at least 53 players on its Active/Inactive List during the entire period of his employment.
(b) A player shall be deemed to have served on a Practice Squad in a season if he has passed the club's physical and been a member of the club's Practice Squad for at least three regular season or postseason games during his first two Practice Squad seasons, and for at least one regular season or postseason game during his third Practice Squad season. (For purposes of this Section, a bye week counts as a game provided that the player is not terminated until after the regular season or postseason weekend in question.)
The Other part of this is is the requirement that anyone signed from another team has to put them on the active roster.
If Markus Kuhn is cut by the Giants and signed by the Chicago Bears he MUST Be on their 53 man roster. Another team can not "block" a player from making the Giants practice squad to put them on their own practice squad they can only put him in the active roster. And once they do that they have to keep him there all year or waive him.
Basically All of the above means that a player can not be on the practice squad if he's been on the practice squad for more than two seasons with the same team OR been on the active roster the season before.
Players who would not be practice squad eligible for the Giants would be like Christian Hopkins or Ryan Perriloux (PS for two years) and anyone else who has been on the Practice squad for two years and players like Spencer Paysinger (on the active roster last year for the whole season), Mark Herzlich etc.
It does get a little more complicated than that, but that's the gist of it (there's a provision where a team can add an extra practice squad player for a foreign player), but that's what you really need to know.
Hope this helps.