Diastatic malt is malted grain, usually barley, dried gently enough to retain starch-breaking enzymes. In small formula-specific amounts it can support yeast fermentation and crust color. Some bread flours already contain malt or added enzymes.
It is not a direct substitute for non-diastatic malt or malted milk powder. Too much enzyme activity can leave bread gummy. Use the quantity specified for the flour and recipe rather than adding it as a general-purpose sweetener.
Reference: Further reading.