Address‐Book Page Modifications
When a store has enabled the OPC's registered-account handling (setting Configuration :: One-Page Checkout Settings :: Enable Account Registration? to true) and a customer completes their account-registration, the customer has created an account … but not yet supplied any address information. When a registered-account holder navigates to their address_book, they're shown a modified version of that page since they've not yet supplied an address. Clicking the "Add Address" button takes them to the normal address-book entry page where they can supply this information.
The language-text associated with the changes is found in includes/languages/english/lang.address_book_register.php and the page-template is found in includes/templates/template_default/templates/tpl_address_book_register.php.