Mill collet question

[DMS]

I think that for ER collets the number us the nominal major diameter in millimeters. The larger the series, the larger the diameter material/tool you can hold. Small collets are common for small, high speed cutters (ER-16/20), and larger collets are more common on larger machines, or for workholding (ER-25 and 40 are common examples). Large collets won't hold really small cutters, small cutters obviously won't hold really large cutters.