What's a rent limit?
Every year HUD, the federal housing department, sets a Fair Market Rent for each area: roughly what a modest apartment rents for there, utilities included. Local housing agencies use it to decide how much a Section 8 housing voucher will pay. Most set their limit, called a payment standard, between 90% and 110% of HUD's figure (federal rule 24 CFR 982.503).
In some metro areas HUD also publishes a figure for each ZIP code, the Small Area Fair Market Rent, because rents differ a lot from one neighborhood to the next.