Second return value of StyleDefinitions in NotebookInformation

Question

I recently found the wonderful "StyleDefinitions" return value from NotebookInformation which I've been using to edit style definitions within a notebook itself.

Generally I use this like:

First["StyleDefinitions" /. NotebookInformation[EvaluationNotebook[]]]

NotebookObject[$FrontEnd, 84]

which returns the stylesheet that I edit programmatically to change my InputNotebook[].

This sometimes has a second return value, though:

Last["StyleDefinitions" /. NotebookInformation[EvaluationNotebook[]]]

{NotebookObject[$FrontEnd, 4]}

which I don't fully understand.

What does it do?

---

Possible explanation

I tend to see this two result form only for stylesheet notebooks:

(*Plain nbs*)
nbsdefs =
  ReplaceAll["StyleDefinitions", NotebookInformation /@ Notebooks[]];
Length /@ nbsdefs

{1, 1, 1, 1, 1, 0}

(*Stylesheet nbs*)
Length /@
 
 ReplaceAll["StyleDefinitions", 
  NotebookInformation /@ 
   DeleteDuplicates[Cases[Flatten@nbsdefs, _NotebookObject]]
  ]

{2, 2, 2, 2}

Where the second argument seems to be the stylesheet notebook it inherits from.

But I'm not certain of this interpretation.

Answer 1

I am now more certain of my possible interpretation.

For instance, try this:

Module[{
  nb =
   CreateDocument[
    Notebook[{
      Cell[StyleData[StyleDefinitions -> "Default.nb"]], 
      Cell[StyleData[
        StyleDefinitions -> 
         FrontEnd`FileName[{"Wolfram"}, "Reference.nb"]]]
      }, StyleDefinitions -> "Default.nb"
     ],
    Visible -> False
    ]
  },
 (NotebookClose[nb]; Lookup[#, "StyleDefinitions"]) &@
  NotebookInformation[nb]
 ]

{NotebookObject[$FrontEnd, 4],
 {NotebookObject[$FrontEnd, 4], NotebookObject[$FrontEnd, 229]}}

So the second argument is telling us which stylesheets this stylesheet is inheriting from