Every developer in San Francisco has to either dedicate units in a development to be made available for buyers that can't afford to buy market rate units.
They can either include them in the building or pay a fee to the Mayor's Office of Housiing that in turn is given to non-profit developers that will build for the low, median and work force incoome levels.
Each development is targeted to a specific income level. The pricing is set according to the area median income, AMI. To buy you need to qualify, to qualify you need to have an income in the bracket they are targeting. Low income is typically below 30% AMI, moderate is 30% to 80% AMI and workforce is 80% to 130% of AMI. The incomes are on the web site that Melanie has given. They track a percentage for single person all the way up to a family of 9.
The draw back is that when you sell and move from the BMR unit it has to be sold at the same percentage of AMI that you bought it at. So instead of market appreciation what you get is appreciation based on income levels rising. It seems to me that this will be a disincentive for movement and not provide the opportunity to move into the market rate housiing.
This situation exists in many areas because there hasn't been enough housing built to keep supplies at a level that can meet demand. Valid question is can San Francisco possibly build enough to meet dmand.
I am very involved in the SF market and work on affordable housing issues with various groups.
Hope that answers your question.