The adapter is considered part of the toilet, and if a regular toilet would fit, the toilet will. Toilets are designed to fit to a toilet flange that sits on TOP of the FINISHED floor (this means, ideally) the flange is sitting on top of the tile and fastened down into the slab through it. Now, many don't end up that way and still work. You might want to use a flange extender, but often a thicker (jumbo or two normal ones stacked) work. Depends somewhat on how recessed it is. Another alternative is to use a waxless adapter - these can accommodate a flange recessed a fair amount.
You'll need a small diamond core or glass bit to drill holes in the tile to mount the Unifit adapter (4 of them). The front part attaches to the existing flange as if it were a toilet, then the toilet gets attached to the back part of the Unifit - so, that end needs to be anchored. They use 4 screws into the floor to do that. A nice side benefit to this method is it's really simple to pull the toilet to paint or whatever since you don't need to mess with a new wax ring...the seal in the toilet to the unifit can withstand numerous insertions/removeals so it's just unbolt, then set back down (well, you do have to undo the water supply connection, too!). really handy if you need the thing inbetween while remodeling and haven't finished yet but don't want it in the way while working.
