Shopify/active_merchant — pull request #1502 by cshepherd (2014-12-24 00:26:30+00:00)
Title: Add PCI Compliant Repeat Billing and Cardholder Information storage for ...

From Netbilling documentation:

This feature enables you to perform a future repeat-billing or up-sell for the same consumer within the same gateway account, rather than storing and specifying the card number and billing address again. Simply check
"Enable PCI Compliant Repeat Billing, Up-selling and Cross-selling" in Step 6 of the Credit Cards setup page. When the feature is enabled, you would send "CS:123456789012" as the card_number, where 123456789012 is
the trans_id of the original transaction.

Per above, netbilling.rb has been modified to accept a string ("123456789012") in lieu of a credit_card for submission.

Furthermore, Netbilling Direct Mode also supports submission of billing information to PCI compliant storage without performing any credit gateway operations with the 'Quasi' (tran_type 'Q') mode. This is implemented in the 'store' method.