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The parts and family system can define different levels for forward, cut, reflected and centerline graphics.
In non-Dynamic View based workflows the Drawing Extraction Manager created new elements in a separate file, this was not a problem. However, Dynamic Views do not create new elements or create separate files, as the same data is just displayed in a different presentation.
The problem is that these alternate presentations cannot be created on levels that do not already exist. Therefore, as a workaround, the dataset is scanned for undefined levels, which are then added to the DGN-file. As a result, users may have levels in their DGN-files that they believe they have not used.
Levels added to the active file are harvested from the Part & Family definitions for Cut Symbology, Forward/Reflected Symbology and Centerline Symbology.
This scan is performed by default to support Dynamic Views.
See Drawing Views [FAQ] (scroll down or search for BB_DVDATASETELEMENTS) for more details.
Managing dgnlib files also suggests how to avoid levels being copied into dgnlibs when you are making changes to them.