From our Cad standards:
1-199 ........ Any solid model including primitive bodies, swept from curves or sketches and sewn solids created from complex free-form bodies.
200 ........... Reference sets for assemblies that are different when after assembly such as a loaded spring.
201-229 .... Reference geometry consisting of sketch curves, wire-frame geometry used to extrude or sweep solid bodies or free-form sheet bodies and datum planes or axes.
230-239 .... Preparations for FEA, CMM, N/C, mold flow
240-246 .... Dimensions, Notes, Labels & GD&T
247-249 .... Customer Borders
250 ........... Title block text,Formats, KPC
251 ........... Revision Column Text
252-253..... New revision columns: After the revisions have gone to the bottom of the page.
254 ........... Revision Block Points to align lines with pattern.
255 ........... Translation data received Models shall have the layers tracked by layer categories.
In assemblies, as mentioned above Item #1 is layer #1, Item #2 is layer #2, etc...
Jon M.