What does Affordable Housing (or sub-market housing) mean in a planning context?

Affordable housing: Social rented, affordable rented and intermediate housing, provided to eligible households whose needs are not met by the market. Eligibility is determined with regard to local incomes and local house prices. Affordable housing should include provisions to remain at an affordable price for future eligible households or for the subsidy to be recycled for alternative affordable housing provision. A full definition is available on page 50 of the National Planning Policy Framework.