A Pattern is a detailed flat image- created by hand or digitally that is used as the template to cut the fabric. The pattern is what dictates fit. Once you have your pattern confirmed(usually after 2-3 prototypes) you will need to grade your file. Grading requires a spec (found in your
tech pack) and establishes sizing. There are industry standards for patterns and grading, but almost all brands take great pride in building garments that fit their target market specifically.