DI is short for de-ionized water same as distilled one. Although all coolants contain inhibitors to prevent scale formation after few years they degrade and you may end up with sludge. All tap water contains dissolved minerals, soft water substitutes calcium with sodium, DI water has almost no minerals. And since the price of DI water is quite low, I think it's a smart choice to use it. If you start flushing and draining using DI there is little chance you'll end up with any minerals in your cooling system. And since you never going to drain all the water adding 50/50 mix will get you different solution at the end.
Happy flushing.