Final answer:
The middle zone in the theory of protection is described as semi-restricted areas, where access is controlled but it is not the most secure area within a facility.
Step-by-step explanation:
When discussing the theory of protection, particularly in relation to security zones, the middle zone is generally referred to as semi-restricted areas. These are areas where access is controlled and limited to persons who have a legitimate reason to be there, but it's not the most secure area. This zone often has measures in place such as key card access or requires identification for entry. Semi-restricted areas are not open to the general public like public access areas, but they are less secure than the core or most secure areas of a facility.