I don't think that all three of your criteria can be conveyed in a single word (i.e. 1. mobility, 2. surface constraint, and 3.
the object itself). You are describing a mobile/moving surface constrained within another (greater) surface.

> (**surficial or coplanar**)
> 
> **"sector", "subsector", "section", or "subsection".**

 
Possibly one of these words would suit the technical purpose you have in mind?

https://www.thefreedictionary.com/surficial

https://www.thefreedictionary.com/coplanar

https://www.thefreedictionary.com/sector

https://www.thefreedictionary.com/subsection