IsReference settings break down like this:
- Not a Reference - ignored by Revit in the project environment, still useful in a family to create "the bones".
Weak Reference - Seen in Project by dimensions, align tool, Revit favors Strong over Weak
Strong Reference - Seen in project by dimensions, align tool and Revit sees these first. (the black lines you see during dimensioning or using the align tool are the weak and strong references in action)
The nice IsReference names like Right, Left, Top, Bottom are all Strong IsReference choices. A reference plane should be named so it is clear why it was created, to you and anyone else later. Programmers comment their code, as much to help them remember why they did what they did as well as what the code is supposed to do. Same concept for these reference planes properties.
The names don't have to match but if the nice names do match what you think they should be, then great. If your families are meant to be interchangeable for other families, like from the stock library you should examine them to see how they are done. I've seen doors that were built opposite when compared with stock Revit doors and when users swap a stock door for theirs, the door panel flips over...ouch.
When Weak and Strong refernces are combined with instance parameters you see grips in the project environment. You can drag the grip to alter the family dynamically. If you use the Align tool on a weak reference the family "moves" toward the alignment edge. If you use the Align tool on a Strong reference the family "stretches" instead.
Last, these help Revit maintain orientation and two intersecting reference planes can define the X/Y origin. One reference plane in elevation can define a vertical origin. Theoretically a reversal of Right and Left reference planes in two different families would flip their orientation if exchanged for one another. Haven't tried that in a really long time so I don't know for sure that condition still exists.
Not finished apparently. Alex quite rightly reminded me in a comment that I forgot a goody! So he wrote:
- "Another advantage of naming your reference planes using the preset Named Planes is that in a project, it will allow you to pick on the dimension grip and bounce between the Left, Right, and Centre options and ditto (depending on orientation) for Front and Rear, Top and Bottom."