You can't find the column to set the Main Contact in any of these objects via Data Workbench:
- Account Contact Persons
- Account Contact Relationship
In Data Workbench, the field for Main Contact is not available.
A workaround for defining Main Contacts is doing it via Migration of Customers, where it is possible to set multiple contacts to an account and define one of them as a Main Conctact by filling the column 'Main' with an 'X' under Contacts tab.
KBA , LOD-CRM-INT-DWB , Data Workbench , How To