I think they test for pollerum and avian flu. Possibly other stuff to. Depending on what they test positive for either you treat it or if it is something bad they will recommend kill the bird and who ever else is infected.
They test for Pullorum and Typhoid, some states add Avian Influenza. That's it unless you pay for more testing, but most do not. Those diseases are not even prevalent in the U.S. nowadays. NPIP was created for the food supply, not to certify backyard flocks as healthy, which it does not do.
You can have Mycoplasmosis testing every quarter if you pay for it, which is what would be of benefit for a backyard flock owner who plans to sell birds or hatching eggs.