The Authorisztion section in the MEX settings allow the restrictions for different Security Groups to be configured at the API level to restrict access to certain API endpoints across all of MEX.
This is different to applying restrictions to a security group via Edit Mode, as making a security group un-authorized to access a particular endpoint will not allow them to use the blocked function at all, no matter where it is in the system.
This function is found by navigating to Settings > Authorizations.
The Authorizations page is split into two separate sections; A list on the left containing all of the security groups in MEX, and a List on the right containing a list of all modules and their authorization status.

Selecting a security group from the list of security groups on the left will display the authorization status for each module in the list of authorizations on the right. The list of Authorizations displays the name of the module in the "Name" column, and the Authorization status in the "Authorized" column.
Each module contains multiple endpoints, which can have their authorizations updated individually in the case that the security group will need some access to certain information from a module, but restrictions in other areas. To view the different endpoints you can do so by clicking the + symbol next to the modules name. This will display the list of each endpoint that can be authorized for the particular module. In the below example, the Department module has had the "Delete Department" endpoint restricted.

This would mean that users belonging to the security group that this restriction applies to would not be able to delete departments from MEX. If a user belonging to that security then attempted to delete a department, then they would get a "Restricted" error message, to inform them that they cannot perform that action.
