This is executed at checkout before calling the sendToPayPal method on the order. The register helper plugin has been updated to update user meta fields during this hook in addition to the pmpro_after_checkout hook. (Because for PayPal Standard, when pmpro_after_checkout is called, the $_SESSION vars are unavailable to it. So other plugins relying on the pmpro_after_checkout hook may have issues with PayPal Standard.)