Gray plastic supply piping installed roughly 1978–1995 that proved failure-prone and was the subject of major class actions. Insurers may surcharge homes that still have it; it's a replace-on-discovery material.
Plumbing terminology in MD, DC, and Northern Virginia maps to older supply lines, basement sump pumps, water pressure regulation, and the licensing (MD PLB & GFC, VA, DC) that real plumbing work requires.