The variety would be the cavity opening diameter in millimetres. ER collets agreement above A selection of 1mm and can be found in 1mm or 0.5mm measures, so A variety of ER collets can hold any cylindrical shank, metric or imperial. ER collets may also be used with a lathe http://cncroutermachine17283.dbblog.net/2498543/an-unbiased-view-of-plunge-router