Both dextrin and chit malts are used to improve body, mouthfeel, and foam stability in beers made with highly modified base malts, but they do it in slightly different ways. Some brewers use them interchangeably at low rates, but they do have different reasons for existing.
Chit malt is lightly germinated and kilned. It’s barely malted, so it retains a lot of high-molecular-weight proteins and unconverted starches. Those intact cell walls and proteins support foam formation and stability, and they also add sort of a subtle graininess/mouthfeel. Because it has its low enzyme activity, chit malt will behave more like a raw adjunct and is typically used at lower inclusion rates (1–5%). It’s a structural contributor. You would use chit malt when you want to reinforce foam and texture in highly attenuated beers (e.g., Pilsners, Kölsch, IPAs).
Dextrin malt (“Carapils” for example) is fully germinated and then gently stewed to convert starch internally before kilning. The result is a malt with a lot of unfermentable dextrins and low in color or flavor. Dextrin malts boost mouthfeel and fullness by increasing residual extract, so essentially, they provide sweetness and body without adding color or flavor. You would use dextrin malt when you want to round out the mouthfeel and sweetness in beers that feel thin or dry (e.g., session ales, lagers, light-bodied styles).
Courtesy of Harmonie Bettenhausen, Ph.D., James B. Beam Institute for KY Spirits, The University of Kentucky