[NTG-context] Overlays and layers documentation?

Hello ConTeXist. Is there any documentation or manual, where I could read some details about using of overlays and layers in ConTeXt? I see on the Internet and wiki ConTeXtgarden some examples, but I would be interested in details relating to the default layers and their names, their order. Are there any restrictions (number, name, etc.) Thanx Jaroslav Hajtmar