Opening ArcGIS Desktop on specific license level?

If you want to control the license on a per-application basis, you could create batch files that set the ESRI_SOFTWARE_CLASS variable to either Professional,Editor,Viewer and then start ArcMap/ArcCatalog.

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A SET ESRI_SOFTWARE_CLASS=Viewer statement will switch to ArcView: enter image description here

And a SET ESRI_SOFTWARE_CLASS=Editor will switch to ArcEditor:
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If you create the batch files, the ESRI_SOFTWARE_CLASS variable is only in scope for that instance of the application. So you could start an ArcView and ArcEditor at the same time (if the license manager has available licenses).

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Alternatively, a user could set a user system variable of ESRI_SOFTWARE_CLASS to the desired license and then every application would be at the same license level.

Related Bug: http://support.esri.com/en/knowledgebase/techarticles/detail/32729