The Special Access tab for a hierarchy unit is where you give users who do not have implicit permissions on a unit special permission to do actions or access information on resources in the unit. Normally only members, managers, or administrators have permissions on units. The exact resource permissions are determined by a Special Access profile that you affiliate with each user when you add them to the Special Access tab.
You must first create a Special Access permission profile before you add a user to the Special Access tab, because you will need to associate the permission profile with user (similar to how you use Team profiles). See Creating Permission Profiles.
Note that you can give users, groups, or units special access to a unit.
Imagine you want to give a user named Hermione permission to see the availability of resources in a unit that is not under her management.
Hermione now has permission to view the availability of members of the "Product Management" unit.