I think, in Oklahoma, it is based on income:

"Although the lease agreement is between the resident and their landlord, THA subsidizes the resident’s rent payments. Therefore, a Housing Assistance Payments (HAP) contract exists between THA and the landlord. The Section 8 Program provides the landlord with guaranteed rent as long as the landlord is in compliance with their HAP contract and the resident is in the unit. For example, if the rent for a three bedroom house is determined to be $550 per month and the client's payment (based on their income) calculates to be $75 per month, then THA pays the difference of $475 per month directly to the landlord."

From (Tulsa Housing Authority example): http://www.tulsahousing.org/HousingO...9/Default.aspx