Google Message Security for Google Apps Administration Guide - Manual (Page 385)

Customizing Notifications 385
Keep in mind that the “stock” notification and the “default” notification are not
the same. The stock notification is what the parent organization supplies one
time only to a sub-organization when that organization is created; whereas
the default notification is the original system-wide template.
They could be the same¨, but if the parent organization modified the default
before the sub-org was created, the sub-org received a copy of the
customized message. Further, by “linking” to another organization, the
notification is updated whenever the other organization makes any changes.
The text customization field in the Edit Notification page:
Scroll down the page to see the other notification options: