Layer and datatype
Beginner
Two numbers on every shape in a chip file. They say which layer it belongs to and what kind of shape it is.
Novice
Two numbers on every shape in a layout file. The layer number says which physical layer the shape belongs to, such as the first metal layer (M1). The datatype separates kinds of shapes on that layer, such as real wires, dummy fill and slots.
Expert
The numbers have no meaning in the file itself; the foundry’s layer map defines them. Mask data preparation usually merges the drawing and fill datatypes into one mask and subtracts the slot datatype, so a shape on the wrong datatype silently drops or adds metal.