What causes the creation of a Default-3D Model?

I do not have a list, but my thought would be anything that creates 3D 'data', such as creating a profile for an alignment.

That's what I would think too. But everything in this dgn was created in SS2 so I can't think of anything that would create 3D data. Plus when I look at the 3D model there is nothing in it.

It is possible that the files were created using a specific seed file that had a 3D model as the Default model along with a 2D model, but the users drew everything in the 2d model.

Or the files originated as a single model 3D file and the user added a 2D model to work in a 2D environment but like you, found they cannot remove the Default 3D model.

A work around is to export the 2D model to a new file and then the 2D model will be the default model in the new file.

I do know that the seed file did not have a 3D model in it. And I did figure a workaround much like the one you mentioned but every time I open it again with SS4 it creates that 3D-model again.