We are offering several sizes here, all cut down from A2 long grain sheets. AN A2 sheet can be cut or folded in two ways. Normally the two short edges are aligned, and the cut is made along the fold to give two A3 sheets. If the two long edges are placed edge to edge and cut the area is the same as an A3, but the shape is much more narrow. We all this a "thin A3". It is the equivalent of two A4s joined at their shorter edges. One of our pictures illustrates this. To differentiate thin A3 from normal A3 were calling the latter "fat A3". Anyone working with fabric will be familiar with this.
Cartridge paper is a strong white machine made paper. It is suitable for all the purposes that office paper is used for, and in most cases slightly better, although not as cheap.