Checkout logic with delivery address, etc.

I'm using eCart 6 and trying to get my head around the checkout page. After using the Checkout Wizard, I now have the pages which make it all up. Checkout.php shows the delivery address with the cart below it. But I want the address details to be populated from the user table but still be able to be updated on the checkout page, or create a new user record if they are not logged in.

Completing the address and clicking 'Continue' causes the cart to slide up and remove the form but I can't see what happens to the data.

Would you be able to give me even just a simple list which shows the logic on that page, what is already performed by the checkout wizard, and what I can and cannot do after that?

Many thanks

Jon