Import loads or merges geometry files that are not 3ds Max scene files.
Merge allows you to load objects from saved scene files into the current scene. You can also use Merge to combine an entire scene with another. This behaves similarly to the XRef Merge dialog.
Replace lets you substitute the geometry of one or more objects in a scene by merging objects with duplicate names. Use Replace when you want to work with less complex geometry to set up your scene and animation, and then replace it with more detailed geometry before rendering.