Ok… what the heck do the values of “Placement” for a window actually do?
I have a Main Window, with a number of custom “Modal” dialog like windows , and have multiple monitors.
I want the dialog to open within the main window (or at least on the same screen). but no matter what I set “Placement” to, it always shows up on the main computer screen, regardless of which screen the apps main window has been moved to
for now I have resorted to this
Me.Left = mainWINDOW.Left+((mainWINDOW.width-Me.width)/2)
Me.top = mainWINDOW.top+((mainWINDOW.height-Me.height)/2)but the drawback is it relies on knowing the “parent” window ahead of time. which for now isn’t a problem, it just goes against my nature to design that way 
In theory Placement should show the modal on which ever screen indicated (same as parent etc)
Most macOS apps have moved away from plain modals to sheets since they are not App modal but window modal and while one is open you can still switch to another window in the same app. If you make those modals into sheets then on macOS they appear attached to whatever window
And on Windows they should also appear over the specific window