When creating a customer, at least one contact’s details must be provided. This contact will be marked as the default contact for that customer.
To change the default contact for a customer, add a second customer and click Save. After you’ve saved their details, you can then specify them as the new default contact for that customer.
Autotask users: when a document is sent to Autotask, only then are new Accounts and Contacts created in Autotask.