Hangar Extensions

Hangar Extensions do not have docking space with doors to accept vessels, but they still can store vessels inside and if you connect them to a hangar correctly, they extend its storage capabilities. The key thing is: each extension and some of the hangars (currently only the Small Inline Hangar supports this) has their stack attach nodes marked as Hangar Passages (see part description in part library):

A ship may only pass from one part to another through a connected pair of such nodes and only if their size allows. So by connecting extensions through Hangar Passages, you may build an extensive and complex storage space with only a single hangar to accept and launch vessels – provided Radial Adapters and Docking Ports (but not the Station Hub) also support this mode, so you may even add storage space by docking or directly transferring a stored vessel from one carrier to another without launching it first.

Here's an example of a ship with extended hangar space. Note that a Hangar Extension is connected to the bottom (green side) node of the Inline Hangar:

Now, if you add some vessel inside through a dedicated GUI window which is accessible through a part menu entry "Edit contents":

You can then open a Vessel Relocation window and move stored vessels between the hangar and the extension:

Note that the parts are highlighted with different colors that match the colors of the selector-buttons in the GUI.