"stage files" vs "cached files"

Hi,

after having read tutorial1+2, I thought the following were more or less 
synonyms: index ~ directory cache ~ staging area

But then I discover that --cache and --staged are two different things, 
notably when using ls-files.

The documentation states indeed:
  "--cached   Show cached files in the output"
  "--stage    Show stage files in the output"

I'm a bit confused. Is maybe a "stage file" entry missing in the glossary?

TIA for clarification/pointer.
Jérôme

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