Today RFC 6197 got published. This document defines the Service List Boundary as an extension for LoST (RFC 5222). LoST allows user to query for the Service List, but would not allow the server to indicate any additional information about the geographical region within which the returned list of services is considered valid.
In order to assist the client in not missing a change in available services when moving, the new Service List Boundary should be evaluated. Only when moving outside the boundary area, a new query has to be placed to the LoST infrastructure since the number of available services might have changed. This ensures that a client is always aweare of all service numbers (especially emergency dialstrings) for its current location.