See the Youtube video for the features of the Vicmarc dovetail jaws:
Designed for mounting bowls by expanding into a recess or clamping onto a spigot.
The sizes indicated on the drawing are when the jaws form a perfect circle. VM120 and VM150 chucks provide approx 48mm of expansion beyond these specified sizes. All Dovetail Jaws have an internal dovetail angle of 14 degrees for clamping and an external dovetail angle of 10 degrees for expanding.