There are certain fields in the different forms of SCSM where you are supposed to add a user or group. These fields are called User Pickers, and if you take a look at the incident form there are three User Pickers on the General tab:
Affected user
Assigned to
Primary owner
Some of these User Pickers target the end users, such as the Affected user field, while others target users or groups from the IT department, such as the Assigned to and Primary owner fields.
When searching for a user to add to the User Picker, SCSM will by default search the whole CMDB for users and groups. This is exactly what we want for the User Pickers targeting the end users, but for the User Pickers targeting the IT department, we are not interested in retrieving any users outside of the IT department.
It's very unlikely that we will assign an incident to a person outside of the IT department for instance. To narrow the scope of the User Pickers targeting the IT department...