19. Your own nodes inside a layer

This is the part of the ship you can rebuild yourself.

Custom nodes inside any sublayer. Open the material in the Shader Editor. Every sublayer is a frame. Add any Blender shader node that does not depend on the camera inside that frame, wire it into the sublayer's chain, and Overpaint treats the result as first-class: it composites it, caches it, bakes it when a bake is needed, and exports it. This works on every content kind, Paint, Fill, Procedural, Colour, Image and Material, and on both buses. Supported node types bake on the GPU baker; a node the baker does not understand falls back to Blender's Cycles bake rather than silently producing a wrong result, and the inspector marks a sublayer whose custom nodes could not be evaluated.

Custom filters and generators without code. Copy a shipped node group, change its nodes, then put it in your library .blend. Use filter_… for filters, generator_… or gen_… for generators. They appear with the shipped stuff after startup.

Keep the copied group's socket names while you experiment (they tell OP how to connect the stack and helper maps). The full socket convention is in the developer reference.

Custom channels are in chapter 18, output shaders in chapter 14. Create Setup Asset packages a stack so you can share it or use it again.