The P is for permanent, the T is for temporary. If you are permanent, you don't have to deal with the service ever again, your retirement percentage will never change. If you are temporary, you have 36 months of retirement benefits while you continue to get treatment for your condition. You have to get reevaluated at least once in those 36 months, and then that documentation goes back to the physical evaluation board for another look at your ratings. On that reevaluation, you can be made permanent or you can be taken out of retirement status. That's probably as simple as I can make it, there are definitely a few details here and there to add.