Grey Prawn, also know as Litopenaeus vannamei is a species largely farmed in Vietnam, China and Thailand. A common species used in Asian cooking. The meat is sweet and tender, ideal for stir-fry, hotpot and steaming.
The size of the prawn is dictated by the number of prawns per kg. In this case, 40/50 means that there are 40 to 50 pieces in 1 kg. Hence the smaller the number, the bigger the prawn.