Contacts to Multiple Accounts versus Account Contact Roles versus junction objects
Key Takeaway
Of Contacts to Multiple Accounts, Account Contact Roles and a custom junction object, only the custom junction object supports record types and field history. Use it when each relationship needs its own record types and field history.
Salesforce offers three ways to link one contact to more than one account: Contacts to Multiple Accounts, Account Contact Roles, and a custom junction object. They differ in how far you can customize them, where their sharing comes from, how they report, and which user interfaces support them. To choose one, match those differences to what the business needs.
The three options
Contacts to Multiple Accounts lets one contact relate to several accounts. The account in the contact's "Account Name" field is its primary account. The link to the primary account is the direct relationship. Every other related account is an indirect relationship.
Contacts to Multiple Accounts stores each link as an Account Contact Relationship record. The IsDirect field on that record marks whether it is the direct relationship or an indirect one.
Account Contact Roles is a standard object that you can reach through the API, but you can barely customize it.
A custom junction object is a custom object with two master-detail relationships that models a many-to-many relationship between any two objects.
A training company has a custom Course object and a custom Instructor object. One course has several instructors, and one instructor teaches several courses. The admin creates a custom Course Assignment object with one master-detail relationship to Course and another to Instructor. Each Course Assignment record links one course to one instructor. One course can now relate to many instructors, and one instructor can relate to many courses.