diff --git a/commerce_shipping.module b/commerce_shipping.module
index f8c9eda..7149b6c 100644
--- a/commerce_shipping.module
+++ b/commerce_shipping.module
@@ -483,6 +483,10 @@ function commerce_shipping_method_collect_rates($method, $order) {
 /**
  * Adds a shipping rate to the given order object for the specified service.
  *
+ * This function is called by commerce_shipping_method_collect_rates() via the
+ * Rules default component for the shipping service,
+ * 'commerce_shipping_service_SERVICE_NAME.
+ *
  * @param $service
  *   The machine-name of the shipping service to rate.
  * @param $order