A frame is the basic structural part: steel plate and gears, made in an Assembler. Frames go into thrusters, containers, control units, field coils and heavy frames; into ships such as the Excavator and Constructor; into stations such as the Port; into facilities such as the Shipyard and Uplink; and into the larger fittings: the cargo bay, heavy drill and reserve tank.
A frame is almost all iron, with a little carbon. The Shipyard is built from frames and motors, so every buildable ship except the Harvester depends on an Assembler.