Unexploded CAD imports slow Revit down and make it more difficult to manage visibility.
The best method is to explode CAD imports and clean them up.
Start new family from Detail Item family template file.
Import CAD file.
Move CAD import so it aligns with the insertion point (intersection of Reference Planes).
Draw any Reference Planes you want to use to make the family parametric.
Explode CAD import.
Select all lines and change sub-category to a Revit sub-category.
From Manage > Additional Settings > Line Style delete any linestyles from the CAD file.
From Manage > Additional Settings > Line Patterns delete any line patterns from the CAD file.
Manage > Purge All.
If you want to create Filled Regions or Masking Regions from CAD import lines:
Select the lines that will make the boundary, CopyClip them (Ctrl C).
Start the Filled Region or Masking Region.
Paste the lines Aligned to Current View.
If required change sub-category of lines.
Finish Masking region.