In our Definity, this is automatically assigned to any VDN that is controlled by an ASAI adjunct link.
If a call comes in to a VDN and the vector then uses an ASAI link to connected to a CTI box, when that box tells the call to be rerouted, the VDN has the Event Notification Adjunct assigned to it.
I'm not sure how it does it, this is just an observation on our PABX.
Cheers