In ArcGIS Desktop 10, a feature class(whether it was in the geometric network or not) could have three fields defined for the symbology unique value renderer as in the following image:
In Pro, for a UN feature class, multiple feature classes that existed in a geometric network in Desktop 10 are now combined into a single UN feature class in Pro. In this scenario, the ASSETGROUP field on the UN feature class layer in a map is automatically used by the symbology renderer definition to denote what used to be the "feature class" in Desktop 10. However, this takes up one of the three symbology fields that can be used for the Symbology definition in Pro. So if three fields were being used for the symbology definition in the Desktop 10 feature class as in the above snapshot, we are not able to add those same three fields to the symbology definition for the UN feature layer in Pro, since one of the three allowed fields are now being taken up by the ASSETGROUP field as discussed above(see the below Pro snapshot). How was this scenario that I have described intended to be handled in Pro?