Revit needs to come up with more comprehensive definitions on curtain wall’s reference plane and offsets of mullions and panels. Currently, there are simply too many variables that can confuse the users. For example, the offset of mullions is described as being measured from panels, but this is not accurate because the panels themselves can be offset from the reference line. It also depends on how the mullion profile is drawn in the profile family; Revit's default rectangular mullion profile, which cannot be accessed for editing, is centered on the curtain wall reference plane. But a custom mullion profile using the profile-mullion family template can be drawn off-centered, such that the center line of front/back can be conceived as the face of glass, if the relationship of the glass and the mullion is so designed.
The confusion arises from the too many variables that are loosely (or wrongly) defined. What Revit should do is to come up with a common rule and eliminate few variables. For example, Revit can eliminate the offset parameter of the mullion profile and instead let the position of the mullion defined in the profile. Or Revit can rule that the face of panel is always be aligned and locked in with the curtain wall reference plane, while the users can control the offset of the mullions. Once the variables are minimized and the rule is set up, it is easy for the users to understand the absolute reference from which every curtain wall elements are measured from.
This will lessen the misguided modelling of the curtain walls which can be prevalent in the work-sharing nature of the Revit model.
