Enabling Contacts with multiple Accounts: how is the primary Account determined?

Question

Just curious: we are considering enabling this feature. We have Contacts that are going to be related to multiple Accounts. Which one will Salesforce pick as the primary Account?

Edit: reading the given answers, I realized that I had made a thinking error. In our org, we have several Contacts that should be a single Contact, but aren't. They share things like name and email address. But right now, each of these Contacts has the Account to which it belongs as its primary Account.

In the future, we must merge these Contacts to get single Contacts and while doing that, we must decide which Account will be the primary one. When enabling the 'Contacts with multiple Accounts' feature, Salesforce will not do any merging, so it doesn't have to decide which Account will be the primary one.

Answer 1

Every contact needs to be associated with a primary account. So on every contact there is an Account field. Any account associated on that contact's field will be treated as primary account for that contact. Any other accounts associated with the contact represent indirect relationships. ie a Junction Object between a Account and Contact called as AccountContactRelationship.

Source: https://trailhead.salesforce.com/en/content/learn/modules/admin_intro_accounts_contacts/admin_intro_accounts_contacts_relationships

https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_accountcontactrelation.htm

Answer 2

I guess you will be enabling the standard salesforce feature 'Related Accounts' to link a contact with multiple account. So there is a field 'Direct' on the Account Contact Relationship object, which indicates the primary account of the contact. The account which will be populated in the contact's account lookup field will have 'Direct' checkbox checked. The account value present in contact's account lookup field should be considered as primary account.