It used to be that when you needed to specify a valve for a particular job, you had to sit down with pencil and paper and do the math to figure out the right size. After you figured out the size, you had to decide which manufacturer and valve material to use.
That was usually done by scrounging around, trying to find manufacturer literature and technical specifications. If you couldn’t find the information and it was outside of normal business hours, you were out of luck.