Find it: Choose File > Import as Inline
When you import an object as an inline, a reference to the file containing the object is added to your scene file. The data within that file is not copied directly. Using inlines reduces file size, but it also places added responsibility on you for managing all the files you use as inlined references.
Use inline objects to break a large file into smaller pieces that can be downloaded separately. An inline object contains the URL for another file, either on the local system or anywhere else on the Web. Some browsers display bounding boxes for inline objects until the complete inline file is loaded. This technique allows users to navigate through a world while they are waiting for all the pieces to load. For complex files, inline objects are an efficient way to manage data for a large number of objects.
When you have imported an object as inline reference, you may want to edit it directly without closing your current scene. The Outline Editor gives you the ability to change your current scene while preserving the last one. If you want to set an Inline node as current scene go to the Outline Editor place your cursor over an Inline node and open the context menu, then select Set As Current Scene. Your last scene is still loaded and can be restored with the Scene Menu above the tree view. Use the small arrow button to quickly change between scenes.
The Scene Menu