I work at a French vocational high school for future woodworkers. I referred your query to one of my coworkers who gave the following explanation:
- débit massif : this is when unprocessed, dried lumber is cut into long planks (using machinery such as with a gang saw) that will eventually be sawn into smaller, more usable boards, etc. Precision is not a factor at this stage of the conversion process. I believe the term is "rough milling" or "rough mill cut-up". You might try searching on Google.
- débit panneaux : this is when processed wood (such as MDF and particle board) is cut to size. Precision is a factor (to the nearest 10th of a millimeter). Once the wood is cut to size, the edges are then veneered, holes are drilled and then the pieces are assembled together. I believe you could translate this as "cutting".

You might want to do some searching on Google to confirm my suggestions.

HTH