userns interface permits a snap to create new user namespaces. This is a system-level interface, and is not designed to be used with higher level snap applications.
This interface supports the inclusion of the appropriate AppArmor and seccomp policies to allow user namespaces to be created when this interface is plugged.
The test code can be found in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/userns_test.go
The source code for the interface is in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/userns.go