Screw speed units

When specifying the recovery/dosing rotational screw speeds some injection molding machines as well as processing datasheets use the circumferential speed (mm/s, m/min, in/s..) whereas others use rotational speed (rpm). While circumferential speed makes more sense, rpm seems to be a popular specification unit for this parameter. Why is that? What screw diameter should I assume from the material process datasheets to make the correction when setting up my specific machine?

What is the most popular unit being used in mother machines?