When would I use Pooled-Random versus Pooled-Static machine types in Citrix XenDesktop 5.5?

Connecting users to a pool of similarly configured virtual machines (VMs) is one way Citrix XenDesktop can create a Virtual Desktop Infrastructure (VDI). XenDesktop further supports two subtypes for these pooled VMs, called Pooled-Random and Pooled-Static. For the former, incoming users are given a random machine from the pool any time they log on. For the latter, each incoming user is always given the same VM. In either case, user state settings don't persist between log ons

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There might be many reasons why you would choose the Pooled-Static over the Pooled-Random subtype, but a primary use case has to do with licensing. Some applications lock their licensing in ways that require a user to always remain on the same VM. In such cases, the Pooled-Static approach ensures applications meet this licensing requirement. For most other cases where pooled machines are desired, the Pooled-Random approach is often the best solution. 

Source: http://www.windowsitpro.com/content1/tabid/57/catpath/virtualization2/topic/pooledrandom-pooledstatic-machine-types-citrix-xendesktop-55-142244