2008

Interface
Merge / Replace Options group
Merge With Current Scene Incoming geometry is merged with any existing
geometry that is already present in the scene. This setting is useful if you have
several separate files that contains components that you want to combine
into a single model.
Completely Replace Current Scene The file you're importing will completely
replace any existing geometry that is in the current scene. If your current
scene has not been saved, you are given the opportunity to save your changes
to the current scene before the import process continues. Default=on.
7010 | Chapter 21 Managing Scenes and Projects