Select two points or edges to create a vertical dimension between them, then choose a position for the dimension text. You can choose lines for the dimension, but they are not actually used. Instead, the endpoint nearest to your selection is used. The selection location determines which side of a circle or ellipse (or its arc) is used. Splines are not selectable for this function, but their endpoints can be used.

Note that the Dimension measures only the distance in the vertical (y-axis) direction. Any horizontal distance is ignored.