The Settings pages allow you to specify various Contacts and Address Books options.
You can specify the name of your Main Contacts Mailbox:
When you use the File vCard operation, the vCard is stored in your Main Contacts Mailbox.
You can specify the name of your Main Address Book:
The Address Book panel displays the Current Address Book. When you login, the Main Address Book is used as the Current Address Book. When you select a different Address Book in that panel, it becomes the Current Address Book.
The Current Address Book is used: