Yes, you can work as a security guard if you hold a Door Supervisor licence, however you cannot work as a Door Supervisor if you only hold a security guard licence. It is somewhat like driving tests.. If you pass your test in an automatic car, you can only ever drive an automatic, where as, if you take your test in a manual car, you can drive both manual and automatic using the same licence.

This in turn makes the Door Supervisor licence and associated course, much more valuable in the real world.