What is a Global Refresh Rate

I’m reading through someone else’s code and there’s a portion I don’t get. I think it’s probably fairly standard because I’ve seen it pop up in other code, but I can’t find an explanation. For reference, win refers to our experiment window:

    introClock = core.Clock()
    while continueRoutine:
        t = introClock.getTime()
        tThisFlip = win.getFutureFlipTime(clock=introClock)
        tThisFlipGlobal = win.getFutureFlipTime(clock=None)

I think I get that nThisFlip represents the expected time of the next screen refresh based upon the introClock. What is a global screen refresh and why is it not tied to a particular clock? I’m inclined to think the fresh rate not just of the window, but the screen, but that doesn’t seem to make sense given that it’s a property of the win object.