Actually, Steve, I think it's a bit more subtle than that. To me, it depends on whether there have been any repeats during the QSO.

If the QSO goes:
CQ RU VE3SMA
VE3SMA VE3KI 599 ON
VE3KI VE3SMA R 599 ON
VE3SMA VE3KI RR73
CQ RU VE3SMA
that seems clear enough to me. If you had not copied my RR73, you would have sent your exchange again, so the fact that you did not repeat your exchange tells me that you are happy with the QSO and have logged it.

However, simple QSOs with no repeats like the above one often don't happen in congested contest conditions. If the QSO has gone:
CQ RU VE3SMA
VE3SMA VE3KI 559 ON
VE3KI VE3SMA R 559 ON
VE3SMA VE3KI 559 ON (meaning I haven't received your exchange yet)
VE3KI VE3SMA R 559 ON
VE3SMA VE3KI RR73
CQ RU VE3SMA, or OH3MA VE3SMA 559 ON
that's not so clear to me. Did you receive my RR73, log the contact, and now are you moving on, or did you fail to receive my RR73 and have you decided to give up and move on without logging the contact and without trying again? How many tries are enough before giving up?

If I have understood correctly, the program provides for a distinction in the second case above, with the TU; OH3MA VE3SMA 559 ON variant message. What seems to be missing is an equivalent for the CQ message, as in TU; CQ RU VE3SMA. Maybe that's already there in the program?? If so, I never saw such a message, not that I was on for very long, so I might have missed it.

There is still some ambiguity if the S&Ping station does not receive and decode the TU; ... message, but it's not possible to remove all ambiguity in all possible sequences without rigid rules about the number of retries. In traditional modes, there are often clues (partial decodes, for example), but in a fixed-length all-or-nothing digital mode, those clues are not there, so either you make the entire process longer in order to reduce the ambiguity, or you decide to live with the ambiguity (and the resulting high busted QSO rate) in the interest of speed.

73,
Rich VE3KI