Concrete Mixes are primarily divided into two different types :
A nominal Mix is generally adopted for small-scale constructions. In this type of mix, the mix ratios and concrete constituent proportions are prefixed and specified. Eg: M20(1:1.5:3); the quantity of cement, sand and aggregate is batched in volume as per the fixed ratio 1:1.5:3. From the above table till M25 grade, the concrete proportions are called Nominal mix concrete.
Design mix concrete is adopted for high-rise constructions. In this type of mix, the mix ratios are decided by an Engineer after analyzing the properties of individual ingredients of concrete. Like, cement is tested for the Fineness modulus and Specific gravity of cement in the lab while deciding the Design mix ratio. There is no Pre-fixed ratio, and ingredients are batched in weight. From the above table, concrete grades more than M25 fall in the Design mix.
In Simple, Design Mix refers to the ratios which are decided by the designer.