I answered a question today about agent VMs and how they relate to VMware HA. I figured it would be worth sharing with all of you. Although there are not many "virtual appliances" using the Agent VM construct I am certain that this will change over time, especially when people realize it is critical to tag them as agent VMs from an availability perspective.
Here's my answer to the original question:
An agent virtual machine is a virtual machine which performs a specific function for the virtual infrastructure. An example of an agent VM would for instance be a vShield Endpoint appliance which is an anti-virus solution. By "tagging" this virtual machine as an agent VM we ensure that this virtual machine gets powered on first in the case of a host failure. We do this to ensure the service is available before all other VMs start.
This is the current order which HA uses in case of a host failure and restarts need to occur:
Agent virtual machines
FT secondary virtual machines
Virtual Machines configured with a restart priority of high,
Virtual Machines configured with a medium restart priority
Virtual Machines configured with a low restart priority
