Image capture - What does "Keep originals" do?

Image capture offers two check boxes when importing from my phone.

One is "Delete after import."

I assume that this means it will delete the images from the phone or camera after they have been imported onto the computer.

The other is "Keep originals."

This is a mystery and I can find no help online.

Does it mean that the originals will be kept on the phone? This would contradict the "Delete after import" selection, even though they can both be selected at the same time.

Or, does it mean that imported pictures will not overwrite earlier versions that have already been imported.

Can anyone clarify this for me?

I like image capture because is allows me control of my image files, unlike iPhoto and its successors. But I am a bit paranoid about losing my photos as I did when I fell into the iCloud trap, deleting all my images when I thought I was saving them.