This is from an NRL email:
"Under the 2017 contracting model, clubs had 25 players to choose from plus an additional 2nd tier salary cap. In the past if clubs needed to use more than this they made application to use additional players.
From 2018, clubs have an additional 5 players (30) to choose but no 2nd tier. Clubs can then make application to use more than 30 players from the development list (up to 6 players) and then players from the 2nd tier competitions (ISP in NSW and ISC in QLD) if they still can't field a team in position from the 30 +6 Development List players."