Thanks for your answers, wesociety!
That is outstanding, yet a little scary.
Where the hell is booktype info stored on the drive?
1) If it is in flash memory, it is a bit scary since Flash memory erases are usually performed by block, not by single address. DVDinfo pro has to readback the block where the booktype info is, modify the block (change the default booktype) and then write back the memory block.
2) If successive firmware flashs don't change the default booktype, either the booktype info is not stored in the firmware flash (but in another non volatile memory), or the firmware flash procedure does not write the whole flash memory!
Either way, I really don't know where booktype info is stored. If someone knows, it would really interest me to know