Frozen Carbonated Beverage, or more commonly 'FCB' is the industry term for what everybody usually refers to as a slushy. Goes by different brand names depending on the store or chain.
For the most part, FCB machines all function in the same way (Slush Puppies being the exception). Syrup and soda water are mixed together at a certain ratio, much in the same way that they do in the nozzle of a post-mix machine (soda fountain). However, the resulting beverage is then fed into a hopper to be cooled, then into a refrigerated mixing barrel to be frozen (sorta), before being dispensed.
Most people I mention this to are surprised that these drinks are actually carbonated. Well they are at first anyway, but they tend to go a little flat sitting around in a mixing drum all that time.
You will almost never see diet-FCBs. This is because the barrel holds the semi-frozen liquid at sub-zero temperatures. The obscenely high concentration of sugar prevents them from freezing solid.
But I did say almost never. Some sugar-free syrup flavors are specially formulated for FCB use. Crystal Light Raspberry, for example, contains propylene glycol to prevent freezing. Food-grade antifreeze. Yum.
Most all of the settings on one of these babies can be tweaked, mostly through a little computer control hidden behind the plastic bezel. Also, the little gadget that mixes the soda and syrup has a manual adjustment. The recommended ratio for FCBs as well as regular soda fountains is 5.5:1 (water to syrup). But this is often a little off depending on the shrewdness/generosity of the management.
Sadly, this requires internal access to the machine, and is done prior to freezing. So, the infamous all syrup super-squishy is beyond the reach of mortal man.
Trust me, that much syrup... not a good idea. You really would think I would have learned my lesson the first time. | 
