If you are coming to Revit MEP from AutoCAD or AutoCAD MEP, you may be very used to specifying a duct system when you draw duct. Revit MEP waits to see what your duct connects to and sets the system based off that information. Connect duct to a diffuser and the duct system becomes supply, connect to a grille and it will become return.
I'm not knocking the system, but if you prefer to do it the old way, or maybe you have unusual systems, here is a way to force things.
A precursor to this is that I prefer not to use systems in Revit MEP at this time. I hope this changes, but right now the benefit is not there for me.
Start by duplicating the duct family you would normally use. I just right click on it in the project browser and select Duplicate.
Now, right click on the new type and select Type Properties. Rename the type to Supply and add the word Supply to its description. We'll use this to control its visibility graphics in the next step.
In visibility graphics edit the filter you use to control the display of supply duct to look for the word Supply in its description not its system type.
That's it. Repeat the process for what ever systems you require. Be aware that fittings will not change visibility graphics, just the ducts themselves. So elbows and transitions will be white where your duct runs are not. Turns out it doesn't really bother me that much because I am able to assign my duct to a system right away and get graphic feed back before the fans and terminals are in place.
It might not be everyones cup of tea, but its an alternative.