Revit has three Detail Levels that can be assigned to a view, which are Coarse, Medium, and Fine and these control how much detail is shown for model elements in the view. When assigning the Detail Level, everything in the view gets assigned that Detail Level by default throughout the view. However, there are times when you want to have different Detail Levels for different categories of model elements in the same view. There can be various reasons for this, but a couple of examples are:
- You want some information to display more detail than others to accent certain items, such as only wanting walls to display the outer wall lines (Coarse display) while information such as furniture in the room shows a high level of detail (Fine display).
- This is common with MEP designers as they often don’t want the walls to show all finishes within the wall. An earlier blog post addresses this situation.
- You want to show a category like Cable Tray with a Fine detail, but show the Conduit as a single line which requires a Coarse or Medium display level.
Revit provides an easy way to accomplish this task.
- Type VG or VV in the desired view to open the Visibility/Graphics Overrides dialog box.
- Scroll down to the category that you desire to be different than the rest of the view and pick in the box that says “By View” in the Detail Level column.
- Pick the arrow button and then select the desired Detail Level for that particular category.
- Pick the OK button to exit the dialog box.
- Enjoy the desired view appearance.
Some examples of how various categories display for system families are as follows:
- Walls show just outer faces of the wall for the Coarse display and all wall components for the Medium and Fine displays.
- Ducts show a single line for the Coarse display and full duct size for Medium and Fine displays.Pipes show a single line for the Coarse and Medium displays and full pipe size for Fine display.
- Conduit show a single line for Coarse and Medium displays and full conduit size for Fine display.
- Cable Trays show a single line for Coarse and full cable tray size for Medium and Fine displays.
- Beams show a single line for the Coarse display and full beam size for Medium and Fine displays.
- Columns show a single line for the Coarse display, web/flange thicknesses for Medium display, and radiuses for Fine display.
Notes:
- This same process can be done in a View Template.
- Loadable families like Doors, Windows, Mechanical Equipment, Electrical Equipment may look the same for each display, depending on how the family creator designed them to look. Revit does not automatically control the display for these like it does for other categories since they are loadable families versus system families.
- System families are created inside Revit project files and are categories such as Walls, Floors, Ceilings, Roofs, Foundations, Pipes, Piping Systems, Conduits, Ducts, among others. The user does not have control over how much detail is actually shown in each Detail View for system families.
- See Revit Help for more information about the Detail Levels.
Applying Technology to Architecture 
“Ducts show a single line for the Coarse display and full duct size for Medium and Fine displays.Pipes show a single line for the Coarse and Medium displays and full pipe size for Fine display.”
This is similar to what I want to do except Id like to change the display based on pressure classification instead of duct size. However, I run into two issues: The first is that the detail level override is only available for the overall “Ducts” category meaning cannot change the detail level of specific subcategories of ducts; it’s all on nothing. The second issue is that even if the previous item was not an issue, you could not change the subcategory of any ducts because they are not editable.
Any comment on how two different detail levels can be obtained for ducts?
I do not know any way to accomplish what you are after with the current versions of Revit.