One of the rules used to create the puzzle's device
A device facet is one of the rules used to create a puzzle. It's considered part of the device.
Assume everything in this reference is a working draft, there's prone to be some mistakes and inconsistencies. I figure it's best to publish and get feedback rather than write for years in secret. The terms will change, the structure will shift, and the bugs will be chased out. It'll take a while.