Reporting on indirect account-contact relationships
Key Takeaway
The Account & Contact standard report type leaves out indirect contacts. To report on every contact related to accounts, create a custom report type with Accounts as the primary object (A) and Contact Relationships as the related object (B).
Contacts to Multiple Accounts lets one contact relate to several accounts. The Account & Contact standard report type doesn't include indirect contacts. So an admin who needs reports on these relationships builds custom report types on the relationship records.
Direct and indirect relationships
A contact's primary account is the account in its "Account Name" field. The link between a contact and its primary account is a direct relationship. Any other account related to the contact is an indirect relationship.
An indirect relationship requires a contact that has a primary account, so a private contact can't be related to other accounts.
You enable Contacts to Multiple Accounts in Setup, not through the Metadata API. The feature works in both Salesforce Classic and Lightning Experience.
When Contacts to Multiple Accounts is enabled, Salesforce creates an Account Contact Relationship record for each contact with a primary account. Salesforce also creates one for each person account.