Delete Imports After Import

I know I'm making this more complicated than it has to be, but, I backup my photos from my iPhone onto my MacBook Air using a cable - yes, a cable - sorry. When I sync my iPhone, I sync it to my MacBook Air with the same cable. Photos starts up as soon as I connect the iPhone to the MacBook Air.

Here's my question:

If I click the "Delete photos after Import" box, from where do the photos disappear - only from the iPhone?

So, when the process goes from importing through Photos to Sync the iPhone to the MacBook Air, the deleted photos/movies/etc. are returned to the iPhone, but do not appear in the Camera Roll?

I have been afraid to click that button just in case the photos are not only deleted from the iPhone, but, incase, through the Syncing process, the deleted photos will somehow notify the MacBook Air to delete them from the MacBook Air's Photos Library.

By not deleting after Import and then Syncing, I get two (or more) duplicate (etc.) photos back on the iPhone, half of which I can delete (There's a trashcan on the page.) and some I can't.

My interpretation of this is that the photos with the trashcan are those that I took or got through iMessage or WhatApp, etc while the one(s) without the Trashcan are those that came to the iPhone through the Syncing process.

Is it safe to Delete After Import?