Hi KVB,Originally posted in cdnusers.org by Kari
In GDSII, every layer has 2 numbers that make up the stream information: the layer number and datatype. In most cases, for major layers, the datatype is usually 0. For example, metal1 drawing would be layer 31, datatype 0. Designers can use the datatypes to organize things how they want. For example, metal1 wires being 31:0, but metal1 fill being layer 31, datatype 5. You will see lots of different usages across different technologies, vendors, etc.
Hope that helps,