And IIRC correctly from training, the screen showed the reservation details but needed another keystroke then to confirm the reservation. Forgetting to do this, combined with handwritten details, was a really common way of causing duplicates.
It's been a long time, but if memory serves it went something like this:
CQA/PADSWI/020518/1215 -
Check
Quota
Availability - In this case on the 1215 from Paddington to Swindon on the 2nd of May - Shows you how many of each quota remain for that journey.
There was then another command you could use to select the appropriate quota straight from that list, or alternatively you could do something like:
CTS/PADSWI/020518/1215/02/OB - Requests 2 seats from Quota OB. For reservations for flexible tickets you'd usually leave the quota off to reserve from the default Standard Class quota on that train.
Assuming you were happy with the seats, you'd then issue:
CTC - This also took the ticket number, name and contact number as optional parameters (Compulsory for Sleeper reservations)
Failure to do that means the reservation wasn't confirmed, and someone could get those seats once it timed out.