This actually simple solution I stumbled upon while working together with Esri Support to define an enhancement for ArcGIS Pro. And it can also apply to other ribbons.
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Default it is not possible to apply transparency on the level of a Discipline or when selecting the added Revit file in the TOC.
When you added a feature layer or made a group layer it is possible to apply transparency. When added a Revit/IFC model to ArcGIS Pro it is possible to apply transparency to Categories in a Discipline. You can select manually one or more Categories and apply a transparency to them. It is inconvenient when working a lot with Revit models in ArcGIS Pro and go over these steps manually every time.
Steps:
1. Select the Revit file in the TOC and right click on the tab ‘Manage’ > ‘Customize the Ribbon’
2. Choose commands from: ‘List of Tabs’:
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3. After customizing the Tab Building, when selecting each level of the added Revit file you can apply a transparency to that level. When you select a category also the ribbon Feature Layer will appear but that is default behavior.
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Success!