When you create a Deal that has several contacts associated with it, you may want to select a specific person or company as your main Contact.
This can be done by marking them the primary Contact for the Deal. In the Deal's detail view, click the star icon next to a Contact's name.
This will move them up on the 'Associated Contacts' list and their contact details, such as email address or telephone number, will be used as the default when making calls or writing emails related to the deal. They will also be listed in the "Associated With" field of the deal.