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 nods, 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 transfer 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.