This is an optional function that stores the unique internal identifier (for those anonymous users). Even when a user logs in to your site, you might also want to identify her by this identifier. This identifier should be called after the initialize call, and can be called even if the initialize call was made anonymously (there are no arguments). Note that if you wish to identify the user by an email address identifier, the initialize call should be used.
_btn.identify("unique-user-id");