Hidden city can be used on a one way or as the last flight of an open jaw. For example, instead of a midweek CLT-ATL roundtrip, buy CLT to ATL, returning to GSO via CLT. It costs halfway between the CLT-ATL roundtrip and ATL-GSO roundtrip fares. Drop the last flight, CLT to GSO.

Or, if you can travel from CLT to GSO one way, you could buy a GSO-ATL roundtrip via CLT and drop the last flight.