Save highlight?

Hey guys, one question I've delayed asking for ages...

What does "save highlight" do?

I mean when you long press over a word or group of words (long-press + drag) you get the context menu where you can "Highlight".

But then if you again long press over a previously highlighted word you can get two options: either "Remove highlight", (obviously) or "Save highlight".

And I don't know:

• When each of them appear. It seems fully random to me.
• What "Save highlight" does.

Any answers, please...