You can use the Draw | Circle command to digitize a circle in Didger. It works in one of two ways: either 2-point mode or 3-point mode. In 2-point mode you click the center and an edge to define the circle. In 3-point mode, you click 3 edge points instead.
Which circle mode is used is controlled in Tools | Project Settings. On the General tab, look for the Circle section that has a checkbox for Use 3 point method.
The Draw | Circle command is documented in the Didger help file.
If you only want part of the circle, first digitize the whole circle and then modify it. There are a couple of different ways to modify it that could be used:
Click on the circle so that it is selected. Convert it to a polyline using the Draw | Polygon to Polyline command. Then use the Draw | Break Polyline command and click where you want to break the circular polyline in two. Delete the parts you don't want. If you want, you can select the remaining half-circular polyline and go to Draw | Polyline to Polygon to convert it back into a polygon.
Digitize a polygon that starts outside of the circle, cuts across the circle where you want to divide it, and then end back outside again. Select the circle and the polygon and use the Draw | Intersect Polygons command to produce a new polygon that is the intersection of the two.
Updated October 20, 2016