The standard series are: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. The "ER" name came from an existing "E" collet (which were a letter series of names) which Rego-Fix modified and appended "R" for "Rego-Fix". The series number is the opening diameter of the tapered receptacle, in millimetres.

Diameter range is the most notable difference between collets and chucks. Generally collets are used on smaller work pieces than typical three-jaw chucks. The general range for collets are 1/16in to 2.5in whereas chucks can be used on work pieces that are larger.

The downside of collets is that they are often less suitable for general use due to the smaller clamping ange - several collets are required to achieve the same range as a standard jaw chuck. Collets are most useful in repetitive work where clamping requirement does not vary significantly.