Flutter: How to understand the `RelativeRect` positioning in the `showMenu`?

Question

I am trying to positioning the context menu in flutter web using showMenu and GestureDetector. I found the way how to do this but the only thing I didn't catch is how to understand the RelativeRect as position? What is actually RelativeRect? My solution is simple:

GestureDetector(
  onSecondaryTap: (details) {
    final offset = details.globalPosition;
    showMenu(
      ...
      position: RelativeRect.fromLTRB(offset.dx,offset.dy,offset.dx,offset.dy),
      ...
    );
  }
)

But I think it is the case-by-case solution. I saw the code for PopupMenuButton and I didn't understand how the starting position is calculated?

Could somebody please to explain?