I've previously been sold an Advance on one coupon with just the origin and destination without any mention of the booked train(s), either on the same CCST coupon or another one. I went back to the station and they issued me a mandatory reservation coupon for the train I actually booked. The danger is that your ticket could be seen as invalid without anything specifying your booked train.
To round off the story ... this is what I did. To be on the safe side, I went to the office I'd bought the ticket from and said that I assumed there should have been a specific train reservation given on/with the ticket. They had a look, and weren't sure what had gone wrong (if anything). When I said what train I was intending to use, they just issued me with a free "seat reservation" (with no specified seat, of course, being SWR) for that train. Though there was nothing at all to link that reservation to the completely separate ticket - there was no sign that they'd put any reference to the ticket it was to go with on the reservation. So would it have really saved the day if I did encounter someone aware of, and wanting to fully enforce, the rules?
But, out of interest, I didn't offer that extra item when my ticket was looked at en route. (My ticket was inspected at several points on the journey, partly because there were changes involved, and partly because my ticket didn't always open barriers, and partly because of routine ticket checks on the train.) On none of these occasions did anyone show any interest in the fact that it should have an associated reservation [perhaps because nothing to that effect was explicitly said on the ticket of, course]; the most that happened was a quick glance at the route and date; and at one stage I got the impression it was a matter of "Oh, it's a piece of orange and white card; I'd better open the gate".
So the end result of this anomaly was that no-one would have minded if I had no reservation; and of course I could have used a train at a different time, with presumably a similar result.