A 3D AutoCAD file imported into Revit is not cutting when sectioned.
The AutoCAD file first needs to be imported into a Family.
You have options:
1. Start the In-Place Mass command and then import the AutoCAD file. This option works well for large, one time imports like surrounding buildings etc. (use the same method for Sketchup files.)
2. Start a Family based upon a Family Template of your choice (ie. specialty equipment) then import the AutoCAD. Load this Family into your Project. This option works well for creating a quick 3D Family with existing 3D CAD geometry.