Deionized distilled water has been put through two different processes: distillation, which boils the water and condenses the steam, and deionization, which pulls charged ions out of the water on exchange resin. The pairing is a laboratory specification rather than a household one, and buying it for the kitchen answers a problem most homes do not have.
Two processes doing two different jobs
Distillation works on boiling point. Anything that will not travel with steam, including salts, metals and most heavy organics, stays behind in the boiler. Deionization works on electrical charge. Water passes over cation and anion resin that swaps dissolved ions for hydrogen and hydroxide, which recombine as water. Running both leaves water with very little dissolved in it at all, which is exactly what an instrument or a rinse tank is specified to receive.
What deionization has in common with your softener
A household water softener is also an ion exchange device, just a selective one. Hardness, as the USGS explains, is dissolved calcium and magnesium, and a softener trades those two ions for sodium on its resin bed rather than trying to remove everything charged in the water. Full deionization removes the lot, which is useless in plumbing: the water is not more drinkable, it just becomes aggressive to the metals and the taste disappears entirely.
Where a household actually meets this water
You will find it in laboratory supply, in electronics rinsing, in some automotive coolant mixing and occasionally in humidifiers where any mineral at all leaves dust on the furniture. What it is not is a treatment for your supply. If the reason you are reading about purity grades is scale on the fixtures or a mineral taste, the fix is on the main line, not in a bottle, and our water softener installation page covers what that work involves.
Questions people ask about deionized distilled water
Is deionized distilled water better than distilled alone?
Only for uses that demand extremely low conductivity, such as laboratory or electronics work. For a steam iron or a humidifier, plain distilled water is already more than good enough.
Can you drink deionized distilled water?
It is not poisonous, but it is not sold or handled as drinking water, it tastes completely flat, and deionization alone does not deal with bacteria or organics.
Does deionizing water soften it?
It removes hardness minerals along with everything else charged, so technically yes, but no home uses deionization for that. A softener does the same job selectively and far more cheaply.