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The ER system is standardised as DIN 6499. This includes chuck, collets and clamping nut.
The big advantage of a collet is the tool or workpiece gets held around its complete perimeter (well nearly).
This is compared to a 3-jaw chuck where the part is held at 3 points.
The result is the part is less likely to move radially.
A collet system is a must for any milling machine.