Not the resolution the display is capable of mind, but the resolution you’re using. If you’re unsure, have a look here: http://whatismyscreenresolution.net
You are missing portrait mode resolutions… I’m really eager to test it, though at home none of my screens supports beeing mounted that way (primary is a laptop, secondaries mount is not tall enough).
At work my IT department is blocking They refuse to change the orientation, as then “no one else were capable of working on that workstation”