Close, but not quite complete. Here is DL's definition of a circle trip, from the
Contract of Carriage.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Circle Trip means any trip, the ultimate destination of which is the point of origin, but which includes a stop at least one other point, or which is not made via the same routing/carrier in both directions.
Examples of Circle Trips:
Example 1: Point 1 to Point 2 on airline A
Point 2 to Point 1 on airline B
Example 2: Point 1 to Point 2 to Point 3 on airline A
Example 3: Point 1 to Point 2 on airline A (first class); Point 2 to Point 1 on airline A or any other airline (coach)</font>
And here is the definition of a stopover:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Stopover means a deliberate interruption of a journey by the passenger, agreed to in advance by the carrier, at a point between the place of departure and the place of destination. Unless otherwise noted, a stopover will occur when a passenger arrives at a point and fails to depart from such point on:
a) The first flight on which space is available, or
b) The flight that will provide for the passenger's earliest arrival at an intermediate or junction point(s) or destination point, via the carrier and class of service as shown on the passenger's ticket, provided however, that in no event will a stopover occur when the passenger departs from the intermediate/junction point on a flight shown in the carrier's official general schedule as departing within four hours after arrival at such point.</font>
So it seems to me that many circle trips include stopovers. No wonder they had trouble answering John's question.