What does alliance infirmary do?

Take in units that exceed infirmary cap

The alliance infirmary is the third and final location for your casualties. Your HQ infirms fill up first, once those are maxed, troops go into your Deep Heal. Once that is full, any overflow troops go into the alliance infirmary. It's capacity is determined by the advancement of your alliance's tech tree research level. We are maxed out in our alliance, so our alliance infirm will hold 100k troops for each alliance member.

It isn't much when you consider players with HQ40+ can house a million troops in their infirms and a million more in their deep heal. You need two things to get your troops out of alliance infirm - alliance coins and either time or healing speedups. If you put 100k in there, you may need a million coins or so to start their healing, and it may he 3 or 4 days to heal.

Depending on how familiar you are with wasteland king, fast comeback, and ace commander, that is where the alliance infirm can be handy. The more casualties you have in WK, the more fast comeback points you earn. If you're playing Ace, that allows you to train more troops at discount and drive your Ace event points up higher.

Be advised - if you go to your state alliance for wk and put troops in that alliance infirm during the event, you need to heal them up before you leave. If you leave the state alliance to go back to your home alliance, any troops left behind in the state's alliance infirm will be permanently lost.