OutlookOwnerApptId - what is that actually for?

Hi community,

I am currently looking into appointment synchronisation issues between CRM Outlook client and CRM 2013 SP1 (on premise).

In the database, I came across the column "OutlookOwnerApptId" in table "Appointment". The Microsoft Docs say it is an integer which is a

"Unique identifier of the Microsoft Office Outlook appointment owner that correlates to the PR_OWNER_APPT_ID MAPI property."

Where is that value coming from and how does it come into effect when CRM Outlook clients syncs with CRM?

I have noticed that some users either have an empty value or "0", others have empty values, "0" OR other values.

Many thanks for you help.

David