Hi During the design of the product, FOS (Factor of Safety) needs to be considered, in normal designer fix a FOS of 1.5 to 3 based on the thump rule. This is not applicable to all dimension and needs to be analyzed carefully. Usaullay, it is the practice that the critical dimension are mating part or any torque / pressure created on the part. When the product is designed for a particular application, engineers are more cautious and tolerance will be tight. Over a period of time, this will be over engineering and value engineering needs to be applied. this is a cycle of over engineering and value engineering. thanks