These are sites which have participated in the cleanup program. Improper disposal such as pouring out the remnants on the ground or letting chemicals go through a sink or shower drain into a septic system can release solvent and vapors. These vapors can rise through the soil into buildings over 100 years after the initial contamination. This can contaminate indoor air and ground water.
Hire a qualified environmental consultant if this layer is showing on a property you are about to purchase.