Is it possible that they were talkng about HotSpring? HS uses an engineered system whereby the tub is supported as well as insulated. They start with a 30# density foam sprayed onto the back of the spa shell by a computer-controlled spray arm in a temperature-controlled environment. This layer is about 3/4" thick and make the shell much more rigid. Then the plumbing is installed (and a bunch of QA steps are taken since they are ISO 9001 certified) but once the plumbing is in place and tested, they spray 10# density foam onto the plumbing to secure it in place AND insulate it for heat and noise.
Then more QA tests and so forth
Finally, once the tub is plumbed and tested, and the cabinet is in place, the cavities are filled with a special mixture of 2# and 1# density foam. Why the mix? They used to get air pockets, and some times that would lead to noise amplification and other minor problems. Now if there are any voids in the 2# foam they will be filled with 1# foam.
Finally, after the bottom of the tub is shaved of excess foam, a layer of 60# density (closed cell polyuerethane) foam is pour over the bottom. This cures hard as a rock. So hard, in fact, that forklifts don't seem to bother it in the least.
As far as I know, other makers simply spray foam into the cabinet.
Density, BTW, is the measure of air in the foam. If you made a once-cubic-foot cube of the foam and weighed it, you would get the density numbers I have mentioned: 1# density foam is very light obviously, but it can settle all by itself, and does nothing to support any plumbing or equipment. 2# is twice as dense, etc. That why the 60# feels about like solid fiberglass.
