This is checked when the contact details are to be synchronised with Outlook Contacts for the currently logged in user. Outlook however is unable to maintain a large list of contacts so therefore the number of contacts which can be synchronised with TriSys is limited.
Any contact details updated in Outlook will be automatically updated in TriSys and vice-versa.
Recruiters typically check this field for their favourite clients with whom they have the most beneficial business relationship.