Layer

A layer is one sheet in a factory drawing, and that sheet holds the shapes for a single kind of work.

The factory does not build the whole picture in one pass. It works one step at a time: grow a film, cut a hole, leave copper, print ink that humans will read. Each step reads its own sheet. If two kinds of work share a sheet, the shop cannot tell them apart, and a drawing that looks tidy on screen becomes an expensive mistake. Open a chip file and turn the colors on and off. A blue family of rectangles might mean "leave metal here." A yellow family of tiny squares might mean "cut a hole through to the sheet below." A strip 2 micrometers wide on the metal sheet is an instruction to the factory. It is not a photograph of metal that can be touched.

The wide rectangle is one sheet. The smaller square is a second sheet, sitting inside the first.

Some files attach a second number to each sheet. That number is a datatype. A named drawing that groups many sheets is a cell. On a printed circuit board the same split shows up as copper, soldermask, and silkscreen. Color itself is a local habit. Teal in one office and orange in another can still be the same factory step when the name and the number match the process kit.

See what a layer is for, what is a layout, and GDS layer and datatype.