When we speak of greenhouses arch, we refer to polytunnel greenhouse model, because the structure of other types of greenhouses is flat, not arched. The shape of the arch is the one that defines the type of greenhouse.
The arch is the top of the truss but sometimes we refer to the truss as Arch name.
The arch is the part of the structure where the roof material is placed, which is mostly made of plastic.
Types of greenhouse arcs: