When placing a dimension to an arc and any other entity the default dimension will be to the centerpoint of the arc. Once the dimension value is accepted, you can drag the end of the witness line to either the minimum or maximum edge of the arc.
You can also right-click the dimension and select Properties. The choices for center, minimum, or maximum will be available where you can toggle the dimension to use any of these settings.
These tricks will only work when the dimension is to an arc. If you dimension to the centerpoint this dimensions to a point in space and does not have these options.