When you set up the directory to sync with Active Directory, specify the user attributes. Before setting up the directory, you can specify which default attributes are required and if needed, additional attributes can be added to map the Active Directory attributes.
Changing the default attributes from a required to non-required and marking an attribute to be required can be done only if there are no directories created. Once the directories are created and synced, they cannot be changed. You can mark the required and non-required attributes before adding any directory in the directories page. When you add new custom attributes after the directories are created, to map them you have to edit the directory and update the directory attribute mapping. The change will be effective when the directory gets synced to Active Directory next time.