Here, the rule is "Mask required unless there is a medical reason why the person cannot wear a mask."
The Catch-22 is that under our federal HIPAA law (Health Insurance Portability and Accountability Act of 1996), no one can ask for or access your medical records, condition, or requirements without a court order.
And under our Americans with Disabilities Act (ADA) no one can refuse service to anyone because of any medical or physical condition.
So everyone realizes that it's not the job of a minimum-wage clerk to "enforce" a state mask order while invoking Federal statutes ... and so no one tries. Saves a lot of confrontation and hassle.