1. Here's why
In Lumion, Imported Models are saved in a special folder that exists outside Scenes.
Every time you add an Imported Model to a Scene, it is a reference to the files in this folder.
What does this mean?
Well, let's say you import a chair from SketchUp.
This model will be saved locally on your PC in the following folder:
Documents/Lumion (Version)/Library
Every time you re-import the chair model, or change its materials/textures in Lumion, the model files in Documents/Lumion (Version)/Library will be overwritten.
So how does this work in practice?
2. Lumion 8.0 and newer:
Example: If you import a green chair, place it in a Scene and save a Scene file (Scene A), a "snapshot" of the current state of the (green) chair will be included in the Scene file.
The green chair is available in the Imported Model browser window and can be added to Scenes.
Let's imagine that you make the chair blue later on and save it in a different Scene file (Scene B).
The blue version of the chair is now available in the Imported Model browser window and can be added to Scenes.
However, if you then choose to load 'Scene A' again, the blue chair in the Imported Model browser window will be overwritten by the original green version of the chair.
3. Lumion 7.5 and older:
There are two different ways of saving Scenes in Lumion 7.5 (and older versions):
You can save a Scene with or without the Imported Model.
3.1: Save Scene function:
If you use the Save Scene function, you are only saving the Scene but not the Imported Models:
Files -> Save Scene and Models tab -> Save Scene
A saved Scene contains all of the model positions, Layer information, the Lumion Terrain, Photo and Movie effects and so on.
Imported Models are automatically saved to a separate folder (Documents/Lumion (Version)/Library) every time you re-import them, or change their materials.
These models are available in the Imported Model browser window in Lumion.
Any changes you make to the model will be reflected in all of the Scenes that you're using the model in.
3.2: Save Scene and Models function:
If you use the Save Scene and Models function, you're saving the Scene and a "snapshot" of the models:
Files -> Save Scene and Models tab -> Save Scene and Models
This 'snapshot' can be transferred to other PCs as a single file (in .LS format).
In other words, the Save Scene and Models function is ideal if you need to transfer a Scene and all of the Imported Models in that Scene to a different PC, or if you just want to make a back-up.
If you load this file, Lumion will tell you if any of the existing model files in My Documents/Lumion (Version)/Library will be overwritten if you go ahead.
Example: If you import a green chair, place it in a Scene and save a Scene file (Scene A), a "snapshot" of the current state of the chair will be included in the Scene file.
The green chair is available in the Imported Model browser window and can be added to Scenes.
Let's imagine that you make the chair blue later on and save it in a different Scene file (Scene B).
The blue version of the chair is now available in the Imported Model browser window and can be added to Scenes.
However, if you then choose to load 'Scene A' again, the blue chair in the Imported Model browser window will be overwritten by the original green version of the chair.