The voice of a person who is not seen on the screen.

The WordReference urlquoted gives the difference between voice-off and voice-over

"off-camera commentary"
Wikipedia gives a term, not so widely used as Voice Over, but still understood :
"Voice-over (also known as off-camera or off-stage commentary) is a production technique where a voice which is not part of the narrative..."
http://en.wikipedia.org/wiki/Voice-over

The idea was probably to use specific terms to make the site interesting, but I also noticed that in the example you have given, it looks like the more commercial type work has been grouped together under "surimpression vocale". The artist (or website writer) is obviously just trying to show variety and distinction here. I would term this as Commercial Voice Over work, whereas the "voix hors champ" section seems like it might be referring more to what VO artists term as "narration". "Narration" describes VO work done for narrative and documentary type recordings, such as a wild life report, or an educational track or programme.

Using the term "commercial" before Voice Over is not essential, but I would call the first, narrative type description as "Off camera commentary" (for the sake of the variety your client obviously wants), and the second (the commercial and mixed media types of jobs) as "Voice Over"(or "Commercial Voice Over). I am sure this is just to avoid repetition because the artists wants to group these two forms (commercial and narrative) into two different sections.

--------------------------------------------------
Note added at 3 hrs (2011-03-05 18:54:01 GMT)
--------------------------------------------------

This website shows the variety of VO work that is normally done. Although there are more than just the two types I have mentioned, ie. commercial and narration, these two terms are a rough division made by industry VOs. For example, if you wanted to group the different types of VO work on a very general level, "animation and character" and "Television Promo" could be viewed as more commercial at times, and "Audio Book" could be classed under "narration" type work.

http://www.theimagingvoice.com/The-different-types-of-voice-...

--------------------------------------------------
Note added at 3 hrs (2011-03-05 18:55:48 GMT)
--------------------------------------------------

"Off camera narration"
You could also replace "...commentary" with "...narration" here I think, although you might not want to if you like the Wikipedia article's terminology!!!

I have always been confused by the French use of 'voix-off. This is from a friend who does this sort of work in France....hope it helps!
"'I've heard "voice overlay" as an expression, when a voice accompanies another voice (which you can still hear behind it), whereas voice-over generally means a replacement voice.... although not always..... I always use "voice-over", even though I do both and work with lots of people who do both...