Metadata update when file is externally changed

[Loekarin]

I need more explanation about the following option:

Settings --> Metadata --> Categories and keywords --> Update when the file is externally changed (date/ size)

-What is meant with 'externally changed'? What counts as 'externally changed'?
Please give examples.
Is this, for example: changed on another computer? Changed in the cloud? Changed in Dropbox? Changed by another user, even when this user used the same computer?

-if checked: Are all changes updated automatically, or only date and size changes?

so, what happens when the keywords (not date, size) are changed of the photo/file on another computer?

-if this is only about date and size, why is this option part of the section about Keywords and categories?