Choose where originals are kept
Keep the untouched originals of edited photos and imported HEIC files inside the library or in a folder of your own — out of sight of a NAS gallery app, say.
The Originals pane decides where the library keeps the originals it holds on your behalf: the untouched file of every edited photo, in an edits folder, and every HEIC converted to JPEG on import, in the same year structure as the library. See The originals folder.
Inside the library, by default
With Keep originals in a separate folder off, the originals are kept in .originals inside the photo folder. Every computer that can see the photos can then see the originals, and a backup or move of the library carries them along. Portman2 never indexes that folder.
Somewhere else
Tick Keep originals in a separate folder and type or browse to a folder, then press Enter. Use this when something else indexes the photo folder — a gallery app on a NAS, say — and would show the originals as extra photos.
Every computer that opens the library must be able to reach the folder, though not necessarily by the same spelling: a mapped drive letter here and the share behind it there are the same folder.
Changing it moves the files
Changing the folder, either way, moves the originals already kept: they are copied, checked, and only then is the setting changed. A dialog shows the move with a way to stop it; Move them starts it. If it is stopped or refused, the setting stays where it was.
Saving an edit needs a reachable originals folder, so that saving never overwrites the only copy.