Firstly, App Volumes is a standard component of the Horizon Enterprise Edition license.
The second option is to purchase App Volumes as a part of the VMware Horizon Application Management Bundle. This bundle includes all the components except the core VDI solution, Horizon View, and is designed to complement existing Citrix environments.
Lastly, you can purchase App Volumes as a standalone product as part of an à la carte list of solutions. Whichever option you choose to purchase App Volumes, the product itself is licensed on a per-concurrent-user basis.