The parameter may be:
|Main or 0||the screen with the primary menu bar (as configured in the Displays preferences).|
|Second||the first from the left which is not the Main screen.|
|Third||the second from the left which is not the Main screen.|
|Internal||the internal screen of the Mac.|
|External||the first from the left which is not an internal screen.|
|Mouse||the screen containing the mouse.|
|Front||the screen containing (the most of) the front window.|
|Back||the first from the left not containing (the most of) the front window.|
|Back2||the second left most screen not containing (the most of) the front window.|
|All||all of the screens, from left to right, one per line.|
|Positive Calculation||the indexed screen from left to right.|
|Negative Calculation||the indexed screen from right to left.|
So for example, %ScreenVisible%Main% gives the visible frame of the main screen.
Like all coordinates in Keyboard Maestro, the frames are from the top left corner of the main screen.