Here a user can send via email a substitution request to another user e.g. to ask him to act as a holiday replacement. The other user can accept or reject the request.

If the other user accepts the request, then during the time of the replacement he will be asked every time after the login, whether he wants to work in the system as a substitute of the requesting user or not. If he wants to work in the "substitute mode", he will be granted all rights which the other user has, except sending a request for substitution.