What are the difference between schedule 40 , 80??

the outside diameter is the same but the difference is in the thickness .

A schedule number indicates the approximate value of theexpression1000 P/S, where P is the service pressure and S is the allowable stress,both expressed in pounds per square inch (psi). The higher the schedule number,the thicker the pipe is. The outside diameter of each pipe size is standardized.Therefore, a particular nominal pipe size will have a different inside diameterdepending upon the schedule number specified.

Its the difference in the wall thickness of the pipe which Is important in the design because it's considering the design pressure and the operating pressure