IN CASE you thought you knew all about Get Functions, Dwayne Wright will set you straight:
This will return a numeric result that will match the number of pixels in the current monitor height. This function can even take into account multiple monitors, which is always a possibility in today’s computing environment. The results of this function are linked to the monitor that contains the foreground window.
It is unlikely but it is possible to have the foreground window covering multiple monitors. In a case like this, FileMaker tries to give you the most accurate information it can by selecting the monitor that has the largest percentage of the window and give you its screen height information.
Lots more at the link: Dwayne Wright PMP, PMI-ACP, CSM – FileMaker Thoughts – FileMaker Get Functions And Layouts.