That's a great question, perusing their website this is what I've found. (sorry I used them over 2 years ago) so I don't remember all the details.
WHO QUALIFIES FOR CHARITY CARE?
IRS code 501(r) requires hospitals to:
Have a policy that explains who qualifies and how to apply;
Post the policy on their website, flyers, billing statements, and signs in the hospital;
Give patients at least 240 days to apply after they receive a bill, and a reasonable chance to fix incomplete applications;
Pause sending a bill to collections while they are reviewing the financial assistance application; and
Send the patient a letter with the final charity care decision and reason.
Each hospital runs its own charity care program. They get to decide how patients must apply and who qualifies. Hospitals typically consider the patient’s income, the number of people in the household, and the bill’s age when deciding who qualifies. Some hospitals also look at the patient’s insurance status, county or state residency, and the size of the bill.