Often times cities (NYC specifically) will impose rules for new housing developments. Referred to as the 80/20 Rule, it requires 20% of the units in a new development to consist of affordable units. These units can't use different materials in construction or be segregated to certain floors. The remaining 80% can be rented at market rates. I realize this doesn't fit under one of your 3 categories, but it is another component of affordable housing.
Often times cities (NYC specifically) will impose rules for new housing developments. Referred to as the 80/20 Rule, it requires 20% of the units in a new development to consist of affordable units. These units can't use different materials in construction or be segregated to certain floors. The remaining 80% can be rented at market rates. I realize this doesn't fit under one of your 3 categories, but it is another component of affordable housing.