There are four restriction options available for the record Sort Key and each of the User Sort keys 1 to 3. They are selected on the Sort Keys tab found on the Options window for each module:

If selected, then the User may pick a Sort Key from the list, amend the list, insert free text entry into the field, re-order the list or delete any values in the list.

If selected, then the User is not allowed to insert free text entries into the field. The User can still select from the list, amend the list, re-order the list or delete any values from the list.

If selected, then the User is only allowed to select a value from the list. They are not allowed to add new values to the existing list, insert free text entries, re-order or delete the values in the list.

If selected, then the User is not allowed to change the values in the record Sort Key field or User Sort Key fields 1 to 3.