As the wiki says, but with a typo of "action" instead of "macro", it is for Keyboard Maestro Engine windows, opened by this macro instance only. I have clarified the wiki.
Here is an example.

The script is:
tell application "Keyboard Maestro Engine"
set x to getvariable "Window"
window id x exists
end tell
The Pause does not have to be 1.5 seconds, but it does have to exist because the action is asynchronous (which it has to be or the next action would not run, and you could not get the token result) and so the window will not necessarily be created and displayed by the time the window is run.