You can read about the device at sctflash.com, the manufacturers website. It's hard to describe what the Xcal can do, because it's so vast. But to sum it up best, it does 3 things very well: 1) it adds in performance tunes, 2) it has user-adjustable options that allow you to custom tailor certain things, like rev limiters, top speed limiters, shift points and firmness, tire size and gear ratio, etc. And 3) it is a monster of a datalogger. Unlike most OBD2 dataloggers (ELM 327 etc), this thing is going to be specific to a manufacturer (Ford or GM, depending on what device you buy), and it is going to have VERY fast sample rates.
Instead of just having generic obd2 data, it will have the manufacturer's "enhanced" data also. This data is much more high resolution, and has a faster sample rate. It also has things in there that aren't covered under the obd2 laws. OBD2 is really just about emissions, and things that would affect emissions. Enhanced data is everything the OEM's got for engine/trans data.
We use these for tuning cars on the dyno, and the software is just great. You can record up to 16 - 20 PIDs or DMRs (sensors). And unlike scan tools that just grab data every couple seconds, this thing does it much much more often. You can blow up the screen to full size, and watch O2 sweeps that are as tall as your computer's monitor! And you can choose how many of the 20 datalogging items you want to see at a time, simply by checking the box next to them. This allows you to overlay as little or as much data as you want over other data.
