The "Global" option would act as if it was a group that contained all the IRIS users.

This option is needed if a resource is to be limited to one use a day, but still giving the user the option to select a interval on the scheduler (As opposed to make a single slot per day).

As of now, the "User" option could be used to not allow an user to book more than once a day, but a second user could still book that day.