A 3d model file is loaded for any chosen text element in the 2d layout window. Z-level and rotation is set accordingly to the text element. A additional rotation can be set in the parameter. The model search path is set in the setup dialog (setup/3d/modelpath).
So for example a text element with the text 'my-model.off' will load the 3d file 'my-model.off' from the folder set in the modelpah and place it the the same location with the same rotation as the text element in the 2d design. The 3d design is hierachical. So if a model is used more than one time, the model will only loaded once and a reference to this model is placed in the 3d design.