Scope means that each widescreen frame is stretched vertically to fill the entire frame. Then it requires a special lens to stretch it back horizontally when viewing. PG and PSB were scope prints stretched in software after fullscreen capture.
Flat can mean either full frame or widescreen with borders, neither of which require a special lens to project.
Scope is generally the preferred type of 16mm or 8mm print if you're looking for the very best quality.