digikam checks the timestamp on the file against its internal library. If the timestamp is different (presumably newer?) it will re-scan that image file. Otherwise, it won’t.
You haven’t elaborated on your reason for wanting to know, so please be advised that this is the behavior I’ve simply assumed as a digikam user. Caveat scantis.
I didn’t want to muddy the water until I had an explanation as to what the option does.
Always before, if I added GPS or Keyword Tags outside of digiKam, the changes were immediately reflected within digiKam.
For the last several months, for reasons unbeknownst to be, I have to do Item > Reread Metadata from Selected Files before digiKam will acknowledge and display the changes.
I have scrutinized Settings and I cannot see any reason for this change in behavior.
I do have the option at the bottom of “Collections” checked, as it has always been.
And, — this is very annoying — if I forget to do the “Reread Metadata” bit and add a keyword tag in digiKam, it erases all other tags that were already associated with the image(s), forcing me to have to remember what tags they were and add them all over again.
I checked that option and it seems to have made digiKam behave as I wanted.
It had always worked as I wished before it quit and I am certain I would not have unchecked it; but, checking it put things back to rights, so it got unchecked somehow.