The layer structure
When a PCB fabricator receives a data set, the first thing he does is to load it into his CAM system to recreate a model of the PCB. That model which will drive manufacturing. To create this model all image files must be converted and the function of each file in the stack must be clearly defined so it is clear which is the top copper layer, which is the solder mask, etc. The layer structure, this is the position of each layer, must be defined - a PCB is not a pile of unrelated images but a strictly ordered set of layers that are laminated together.
The goal is to define this layer structure in an automatic process following a defined standard to eliminate manual work and subjective interpretation. This is precisely what the Gerber specification's
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