Exploring the Contacts to multiple Accounts feature
As you would expect, the Contacts to multiple Accounts feature allows a Contact record to be associated with more than one Account record. When Contacts to multiple Accounts is enabled, Contact records essentially have a primary Account record relationship and zero or more indirect Account relationships.
Important note
Disabling the Contacts to multiple Accounts feature will effectively delete all indirect Account relationships for your Contact records.
When enabled, the Account object’s standard Contacts-related list can be replaced on page layouts with the Related Contacts list instead. Contact page layouts can have the Related Accounts list added to them. Person Accounts can have both related lists added to their page layouts.
Important note
By default, any activities associated with a Contact record will roll up to the primary Account record for the contact. It is possible to disable this default behavior...