What does "merge" in ido.el mean?

[Xue Fuqiao]

I'm reading ido.el, and I have a small question.

I saw the word (or term) "merge" in in `ido-ignore-directories-merge',
`ido-merge-ftp-work-directories',
`ido-auto-merge-work-directories-length', and some other
symbols/docstrings.

I don't know what this term means here (what does it merge?).  I want
to read the sources in detial but don't know where to start.  And I
have also searched "emacs ido merge" and "ftp merge" on web but
there's nothing promising.  Can anyone point me in the right
direction?

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Best regards, Xue Fuqiao.