The ERD is the diameter of the rim measured where the nipples are seated in the spoke holes, with the thickness of the two nipple heads added on. ERD is a key measurement in figuring spoke length for a wheel build. The bead seat diameter (559 for a 26" mountain bike rim, for example) is the diameter of where the tire's bead seats on the rim. The ETRTO system lists the bead seat diameter and width of the rim, for rims; bead seat diameter and width of the tire, for tires. The ETRTO system is for rim/tire sizing, ERD is a number used in spoke length calculations.