It would be nice to see all the different frankings out there. I have an envelope from Vietnam which only needed to be franked "FREE" where the stamp woid be; but it is not just free written on it, I made a stamp with a decorative FREE on it along with some squiggly lines. Didn't have glue so I went to the O Club and got a dab of molasses to use as glue!
If I get the gumption to look for that franking, I'll post a pic of it.
Porto (portare: to carry) was the money due on carriage of mail. This used to be paid by the recipient. See, e.q., Dutch postage due stanps inscribed "PORT."
The word frank derives from francare: to make free. The carriage of mail is made free of porto (franco di porto) by prepaying postage. The recipient thus does not have to pay on receipt as was normal in the past. The proof of franking the mail, the frank, was a stamp or label (now postage stamp).
A frank is proof the recipient is "franked" of paying for carriage of postage.
So, yes, an embossing of a cover that proofs prepayment is a frank.