Because named permission sets can vary from computer to computer, the use of named permission sets for declarative security is restricted to the immutable named permission sets included as part of the default policy. This ensures that the permissions contained in the permission set referenced will be the same wherever the code is run. A compiler error will be thrown if a mutable or unrecognized named permission set is used.

Namespace: System.Security.Permissions
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0