Klaus, I read carefully that section of Ralph & Chriss book myself. I just had another read.
The section at the start on page 132 is about paper black density, where you take a blank negative and make density tests to get an effective exposure time for your personal zone 0 with that film and paper/enlarger set-up.
With that blank negative the actual rebate and the negative whole, are one and the same.
Normally a negative has information on it in the area that has been exposed, but the rebate, or edge, has the same exposure as this blank test negative, none.
The text refers to the picture 2a and 2b in the context of the film rebate. 2a was printed using the film rebate/clear negative time, as the correct exposure for a print, this was too dark.
Picture 2b was printed at a different exposure to make the best possible print. To do this, the film rebate (clear part of the film edge) was ignored. The resulting picture is better.
Picture 2c (the final picture in the series) illustrates how the whole lot is better, by rating the film at a different (slower) effective speed.
Ralph Lambrecht comes across as concise, precise and technically careful, in the language used to describe the processes and ways of photography, you have a very good book from which to learn more from.
Rebate in English usually means one of two things. Firstly and usually, it means a discount on a purchase.
Secondly and not that usually, it means a wood working term where you cut a groove or edge in some timber to either mate two pieces of timber or to form a short edge, like the runner on a drawer. The Americans usually refer to this as a, rabbet.
I hope this helps a bit.
Mick.
Ps:- I used my Lexicon, as oposed to my Luxikon

Good choice of name!