A: A Remote Desktop Services (RDS) Client Access License (CAL) is needed whenever any component of the Remote Desktop Service (or formally, Terminal Services) role is used. This could be an actual Remote Desktop Session Host (via Terminal Server), it could be RemoteFX running on Hyper-V attached to a virtual machine (VM), the RD Connection Broker, any component.
The two main Citrix solutions I want to focus on are XenApp and XenDesktop.
XenApp is a session virtualization technology that delivers either a desktop or just an application that's running on a remote server OS. Because XenApp is built on RDS, the RDS CAL is required for every device or user that uses XenApp.
XenDesktop is Citrix's Virtual Desktop Infrastructure (VDI) solution that uses pure Citrix components. No RDS role services are used, which means that for a pure XenDesktop solution, the RDS CAL isn't required.
But there is a caveat. While XenDesktop doesn't require the RDS CAL, it's very common that XenDesktop deployments also use XenApp to deliver applications to the VDI desktops. If the XenApp capability is used with XenDesktop, then once again the RDS CAL is required.
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