When you get to the review of wosc-meta-util.php you will come across two important functions.

The first of these functions is *_wpsc_validate_visitor_meta_key( $visitor_meta_key )*.  This function is called within each of the visitor meta key access function to make sure the meta key being accessed is ok to use.  The function will return the validated meta key.  

In cases where a meta key has been aliased to a different meta key, the target meta key will be returned.  That means if you want to set the shipping same as billing flag to true using *shipping_same_as_billing* as the meta key, the returned validated meta key would be *shippingSameBilling*.  

The target values are defined in the *_wpsc_visitor_meta_key_replacements* function.  The initial mapping is based on the various meta keys that were found in the code and in the database of a site that has been running for a period of time.  A filter hook is provided so that devs can add their own substitutions as their plugins and themes evolve.

The target names are set to match the unique name field defined in the checkout forms data.  This gives us the ability to map the user interface that needs to be presented to any meta item value that needs to be retrieved, and vice versa.  Additional meta item names can be any valid WordPress meta key as there isn't a conflict with the unique names we have defined.

There are code comments to explain all of the above.