I have replied to your previous post (https://forums.autodesk.com/t5/fabrication-cadmep-forum/how-setup-camduct-to-work-with-ductboard/td-...) also but I'll repeat it here as well.

To get the layers as you want them when exporting a nest as dxf:

You need to setup the DXF Output Layer Mapping to map CAMducts layers to the dxf file layers.

This is done in CAMduct via "Database - Manufacturing - Tool Defaults - Layer Mapping - Export"

See attached screen shots