I have not yet rented to Section 8 tenants, but have had so many inquiries from people with vouchers that I did investigate some of the details. This may or may not apply to any area other than my county.
The Section 8 tenant gets a voucher for a specific number of bedrooms. Depending on the area / neighborhood, that voucher gets a certain dollar value. The landlord can receive that full voucher amount if the landlord is paying for all utilities! No way for me to pay all utilities, so next comes handling of utilities. From that voucher amount, the Housing Authority subtracts the various utility allowances that end up being the tenant's responsibility, along with charges for the tenant needing to supply their own refrigerator; those utility allowances are re-calculated every year, as are the fair market rents. That's how the landlord can determine the "net" rent they will receive.
Now, the caseworkers are assigned to tenants, not to landlords, so getting cooperation from the Housing Authority can prove to be a challenge. They really don't want landlords to know the numbers for the fair market rent and utility allowances, because the landlords will manipulate the asking rent to take as big a "net" rent as they can - counter to the Housing Authority wanting to restrain amounts paid out.
So, you can have the tenant applicant bring all of the information from the Housing Authority if you really want to learn what the amounts are. Tell the applicants to get a copy of the utility allowances (they will already have their voucher amount) so that you can determine whether renting to them will pay you enough.
Then there is the matter that Bill mentioned, where the tenant can make up some of the rent from their income. The Housing Authority says that a tenant should pay 30% of their income toward rent, and that in some circumstance a tenant may be allowed to pay up to 40% of their income toward rent. In Bill's example of $900 rent and $800 allowance, if the tenant has at least $1000 income per month (approximately), then that might work. Where did the $1000 come from? $900 - $800 = $100 is pretty easy to see. Then that difference has to come from the 40% - 30% of income, so $100 in this case has to be no more than 10% of income.