There are different definitions of "affordable housing." One apartment project in Plano is described in The Dallas Morning News: "The development would include 128 units set aside for households at or below 60% of the area’s median income. The remaining units would have no income restrictions. The median income for the city is approximately $95,000 a year."
More important than the exact definition (for me) is that at least Plano has some developments with a target for affordability. Any project that comes before Richardson's CPC or City Council seeking a zoning change should be required to meet a target as well.