When creating or placing a civil cell, what is an "Optional Reference"?
Once you have pressed reset to finish selecting reference elements, you have the option of selecting optional references. Optional references are reference elements that may, or may not, exist when you place the civil cell. If they exist, you can select them and the supplied geometry will be used. If they do not exist, the civil cell will create them using the predefined geometric rules captured during creation.
An example of this might be a side road where you want to create an intersection. If the length of the side road is significant, you might have already designed the full road corridor, including the road edges. If the side road is less significant, you may have only designed the centerline for it. The civil cell of the intersection could be created so that the side road centerline is a reference element, because it exists in both cases, and the road edges for the side road could be optional references. When you place this civil cell on the full side road corridor you would select the optional side road references, because you have already designed them so you will want to use them. When you place the civil cell on the side road centerline you will press reset when you asked to select the optional references, and they will be created by the civil cell.