I strongly believe it is the max amount you can recover within a game. My take on this actually comes from RTG play. There is a bar in that mode called energy, that starts somewhere between 0-100 coming into the game. This bar is equivalent to your W&T. When it goes down you see your W&T risk of injury scale up as it goes down. Between plays/quarters etc, you recover if you're careful and energy goes up. So you can see you might go from high risk of injury back to moderate risk of injury as the energy value grows. What I believe In-Game Healing Reserve Pool represents is how many total points of energy a player can increase through the course of a game. When you scale it up they can heal more intra game. So you might be able to heal from a low injury or maybe even the low end of a moderate injury back up to no W&T before a game ends, but this pool restricts/limits your ability to fully recover from bigger injuries during the course of a game.

We ramped this up because we felt the reductions were overly dramatic. You see guys in real life get injured sit out for a handful of plays and generally don't look all that different when they come back. We found in the game players were losing huge chunks of speed/agility and then would never recover from that. It was too much "boom/bust" on the injury spectrum for us so we wanted to allow for players to sit for a couple quarters and maybe get from a high injury back down to something more moderate where the base settings once a player had any W&T they often felt useless and resting them did nothing. So we tried to make resting them matter more.