This viewer adds new "collision bones" to the standard avatar skeleton. Properly rigging mesh objects to those bones will cause the garment to adapt to changes in the avatar shape made using the avatar editor sliders and avatar physics.
At this time, the new skeleton should be considered provisional and subject to change; we do not yet recommend selling or buying garments rigged to it. Since we may find reasons to improve it during this testing process, and any change to the collision bones will likely break garments rigged before the change, we want to make sure that we have a set of bones that we can all live with into the indefinite future before it is any more widely used.
Creators and other users who participate in testing can provide feedback by filing Jira issues in the FITMESH project.