When you install the VDA on a session host, you have to enter a list of your delivery controllers one by one. This list is populated in the registry:
The value will be equal to a list of your delivery controllers, each seperated with a space. e.g.
XENAPP01.homelab.com XENAPP02.homelab.com XENAPP03.homelab.com
You can also set the list of Delivery Controllers via Group Policy:
This can be used to re-enforce the DDC list set during the install and also as a handy way to add or change delivery controllers without needing to update your image.
Note: You will only see this policy option from Group Policy management run on one of your Citrix 7.x Delivery Controllers. If you set this and then try to view the policy from a different machine that doesn't have the policy, it will show up empty.