In order to understand practice squad eligibility rules, you have to know what "accrued seasons" toward free agency are. To accrue a season, a player has to be on an NFL regular roster of 53, an inactive PUP list (regular season version of PUP) due to an NFL football related injury, or an IR list for at least 6 regular season games. Players who have not already "burned through" their up to 3 seasons of PS eligibility, clear waivers, and have zero accrued seasons, or have been on a game day active roster of 46 for no more than 8 games total during the accrued seasons they have accumulated toward free agency are PS eligible. A team may also sign up to 4 out of 10 players to their PS who have no more than 2 accrued seasons, regardless of how many regular season games they were on an active game day roster of 46 ("special" PS slots). So basically any player who was drafted or signed as a CFA in 2014, 2015, or 2016 is potentially PS eligible if he gets waived and clears waivers.
To determine the potential PS eligibility of former PS players, it is necessary to determine how many seasons of PS eligibility they have "burned through." For the first 2 seasons of PS eligibility, a player "burns through" a season of eligibility by being on a team's or multiple teams' PS for 6 or more regular and/or post-season games. For a third and final season of PS eligibility, a player only has to be on any team's PS roster for one regular or post-season game. If he gets released from a team's PS during that 3rd season, he can be re-signed to that team's or some other team's PS that season, but he won't be PS eligible in future seasons.
Other PS rules and facts to take note of:
Current rules (2016) dictate that teams can have up to 10 players on their PS rosters. (Original rules only allowed for 8). Of those 10 PS slots, up to 4 can be used on any player with no more than 2 accrued NFL seasons ("special" slots) regardless of how many regular season games a player has been on a game day active roster of 46.
Players on the PS roster (for the 2016 season) get paid a minimum of $6900 per week (bye weeks count), for each regular season and postseason week they're on the PS roster, and it counts against the cap. There is no maximum allowed PS salary, but as I said, PS salaries count against the cap, so PS players are rarely paid more than the minimum.
If a PS player gets promoted to his own team's regular roster temporarily, and his team needs his roster slot, he must clear waivers again before he can re-sign with his current team's PS, or sign with any other team's PS.
PS players are "quasi" free agents. Team B can offer Team A's PS player a minimum salary contract for their regular roster of 53, and if the player accepts, team A just had their PS player "poached." However team B must pay team A's former PS player that minimum regular season roster salary for at least 3 weeks, and the player will take up one of team B's 53 regular roster slots for at least 3 weeks, even if he is released before the end of the 3 weeks.
On to the list of players who were on the Broncos' TC roster of 90 who will be PS eligible IF they clear waivers. I will list the players by positional group. An * indicates a player who has not yet been been released by the Broncos at the time I am writing this post, so still may have a shot of making the roster. I will omit any players who are generally considered to be roster locks.
Mathu Gibson, Cameron Jefferson (OT), Kyle Roberts (OT), Lars Hanson (OT), Robert Myers, Aaron Murray, Dillon Day (C)*, James Ferentz (C)* (made initial 53), Darrion Weems* (made initial 53), Aaron Neary*.
Manasseh Garner, Henry Krieger-Coble*, Jeff Heuerman (made initial 53)*.
Bralon Addison, Durron Neal, Kalif Raymond, Mose Frazier, Jordan "Sunshine" Taylor (made initial 53)*, Bennie Fowler (made initial 53)* (special), Cody Latimer (made initial 53)* (special).
Juwan Thompson (special), Kapri Bibbs (made initial 53)*.
Calvin Hertelou, Kyle Peko (DT) (made initial 53), Lars Koht, David Moala (NT).
Frank Shannon, Darnell Sankey, Dwayne Norman, Zaire Anderson (made initial 53)*, Corey Nelson (made initial53)* (special).
Eddie Yarbrough, Sadat Sulleyman, Vontarrius Dora, Kyle Kragen*.
Antonio Glover, Ryan Murphy*.
John Tidwell, Taurean Nixon, B.J. Lowery*, Lorenzo Doss (made initial 53)*.