Calcium Chloride is needed for cheese makiing when you are using store bought milk that has been pasteurized.  Some cheese makers still use it with raw milk. It's purpose is to help restore the milks calcium that was lost through the pasteurization process and in turn this will give  a stonger curd during the cheese making process. It is vital when using any pasteurized milk.
46 gram bottle . Use 1/4tsp. per 4 to 6 litres of milk ( 10 or more batches)