Non-diastatic malt has been heated so its starch-breaking enzymes are inactive. Bakers use it for malty flavor, sweetness and color in foods such as bagels and breads. It may be sold as a powder or syrup.
Diastatic malt performs an additional enzymatic job and cannot always replace it. Syrup also adds water, and malted milk powder contains other ingredients, so the product form matters when making substitutions.
Reference: Further reading.