In Citrix, multiple copies of a process (e.g. ArcMap application) are run on a Citrix server.
Therefore Concurrent Use is supported as the only licensing method in Citrix environments.
License switching for a single user with ArcGIS Administrator is then not possible.
Instead, the environment variable ESRI_SOFTWARE_CLASS is used:
The license selection/switching with this variable works of course also in "normal" Windows environments.