Extend Membership Registration Form in WordPress


I'd like to extend the registration form in WordPress. I've done extensive searching in the community and all I can find is: (1) how to do it in BuddyPress and (2) that it requires "custom coding" in WP.

I'm fine dealing with the custom coding, but I have a bit of confusion over this line in popover_signup.form.php:

<?php do_action('membership_popover_extend_registration_form'); ?>

This suggests to me that this section of the page would pull in extended registration form fields. Is that the case? If so, how does it work? If not, what does this do?