Layer map
Beginner
A lookup table that says which number in the chip file means which layer.
Novice
A table, set by the foundry for its process, from layer names and purposes (M1 wires, M1 fill, M1 pin labels) to the layer and datatype numbers used in the file, such as 15/0 and 15/1.
Expert
Every tool that writes or reads the layout must use the same map: the place-and-route export, IP deliveries, the fill tool, and the DRC and LVS rule files. A mismatch is a classic cause of a broken merge.