Microsoft provides two forms of volume license activation for Vista: Key Management Server (KMS) and Multiple Activation Key (MAK). The choice of which form to use depends on whether Citrix standard-image mode is needed.
KMS Model: KMS volume license can be used with Provisioning Server for Desktops in either Private Image or Standard Image Mode. Clients communicate to a central Key Management Server. Client machines identify themselves to the KMS by sending a hard-drive signature ID to the KMS.
MAK Model: MAK activation, which requires that key and activation information be created and stored uniquely on each client (target device), is only compatible with Provisioning Server for Desktops in Private Image Mode, and only when each vDisk is built uniquely for each machine (vDisks cannot be cloned). Because of these limitations, use of MAK’s for machines that are served by Provisioning Server for Desktops, is not recommended.
ref: http://support.citrix.com/article/CTX124106
