xPDK Layout basics

layer

This section allows to define a list of GDS layers. These will be written to GDS or Oasis file or OpenAccess database.
A layout oriented PDK should have one or more of these therefore.
Per layer single-layer operations are available (DRC, derived), see single layer.

applayer

This section allows to define a list of applayers. These will not be written to any output, but are typically needed for supporting DRC.
Per layer single-layer operations are available (DRC, derived), see single layer.

MultiLayerOps

This section is for multi-layer (GDS or app) operations. See multi layer.
A basic design kit does not need this section, but when a series of (gds or app) layers exists, it is likely to be needed to describe more complex rules and define inter-layer aspects.

xsection

This defines a xsection which in EDA-design is often called a cad or design layer. In OptoDesigner this is a mcs.
A layout oriented PDK should have one or more of these therefore and they are key for schematic / circuit design also, as connecting ports in a schematic should have a matching xsection.

extend

Allows to extend with any (complex) data for enriching a design kit. The sub-data of an extend will normally be vendor or provider specific, but may span multiple software vendors or suppliers.
Provider extensions can be references to (or embedded) reporting scripts or their versions; source database references and so on. Such data is helpful in cross-referencing production issues or differences between export snapshots. Embedding such data in the XML rather then side-files enhances tracebility and reduces errors.

More details on extensions naming & conventions.
For OptoDesigner specific extensions are defined at main level (so outside the extend) see OptoDesigner extensions.