This is an echo of a post from October 2011. I emphasize this concept in every session I do that is focused on Revit MEP and I think it's worth stating again.
Revit MEP elements, like electrical panels and receptacles or HVAC equipment and diffusers, have a Parent - Child relationship. Revit calls this a Logical Relationship. In contrast the physical relationship is described with duct/pipe but there is no equal for electrical systems. It could be with conduit or cable tray but no such relationship exists yet.
The other day we were chatting about this in class and I blurted out "You know, like who's your Daddy?" I was kidding but one of the guys said that it will actually help him remember to start with the "child" part of the relationship. For example, you start with a receptacle and create a power circuit (system), then choose the Daddy, the electrical panel it gets power from. This relationship continues up through the grandparents, great grandparents etc. When everything is assigned correctly you can see this family tree in the System Browser.
So if it helps, when you are creating relationships between elements with Revit MEP, just remember "Who's your Daddy"! START with the child and then assign the Daddy.