This synchronization process is defined in the pipeline batch Full CRM Contacts to xDB Sync Pipeline Batch.
|Pipeline batch||Contact sync||Marketing list sync|
|CRM Contacts to xDB Sync||yes||no|
|CRM Lists to xDB Sync||no||yes|
|Full CRM Contacts to xDB Sync||yes||yes|
Both CRM Contact Synchronization and CRM Marketing List Synchronization update xDB Contacts. Both of these processes update contacts work by storing the updated contacts in a work queue, and only updating xDB after all of the contacts are read. This reduces the number of times you have to explicitly write to xDB, which may improve performance.
The full contact synchronization process uses this same approach. It only updates xDB after the contacts are synchronized, and the marketing lists are synchronized. Again, the benefit is that, in general, the fewer times you have to write to xDB, the better the performance on your Sitecore servers.