-cust_bill, svc_acct, cust_pay, or cust_pay_pending object).
+cust_bill, svc_acct, cust_pay, or cust_pay_pending). If the object
+is a svc_acct, its cust_pkg will be fetched and used for substitution.
+
+As a special case, this may be an arrayref of two objects. Both
+objects will be available for substitution, with their field names
+prefixed with 'new_' and 'old_' respectively. This is used in the
+rt_ticket export when exporting "replace" events.