Yes, similar to that...IR is only for players on the active roster. There essentially is a practice squad version of that called Practice Squad/Injured where they can be put and they don't count against the PS roster until they are activated from it. Follows the same rules as IR where they have a 3 week ramp up period where they are allowed to practice prior to being activated but unlike the IR, they don't count towards your limit of 8 IR activations per season as someone else pointed out upthread.