Fischer is saying that with its heat and compression "Vacuum" process, the shells for that model can expand/compress within the range of 91mm-98mm. Fischer pioneered the "Vacuum" technology several seasons ago and has since continued to refine it by making improvements to the liners, compression/expansion process and some other minor tweaks.
Basically the shells are heated in an oven and then your feet are inserted and compression pressure is dialed in to your specifications after discussing your needs with the boot tech. The compression can be set very high to provide a very, very close fit or less so for a more comfort oriented fit. Your feet push against the heated plastic where more room is needed and the compression "shrinks" the plastic to conform to spaces and voids in your feet and lower legs to the shell top.
The amount of forward lean can also be adjusted by several degrees for more upright or forward leaning boots. The net result is a pair of shells that conform to the individual peculiarities of your feet/ lower legs with customized forward lean.The process can also deal with canting issues.The shells are then cooled down to set their permanent shape with your feet still in them.
To do a Fischer Vacuum boot well you still need a skilled boot fitter as it is not an "idiot proof" process. Last season, I had a new pair of Fischer RC4 Vacuum 130 shells done by Corty Lawrence at Footloose Sports a Mammoth Lakes.
I also have a pair of the first generation RC4 Vacuum 130 boots. The one change that I have made is to replace the stock liners with Intuition liners because I find them more toasty.
Yes, the The RC4 Vacuums can still be ground out and otherwise customized, if needed. The "Vacuum" process takes a lot of trial and error out the the bootfitting process and can provide a very precise fit. Several other boot companies have developed their own new versions of customization including Salomon and Atomic and I'm guessing others.
I'm not affiliated with Fischer so there may be a Fischer Rep here that can add more.
Edited by Lostboy - 8/27/16 at 8:22pm