For me discipline is different that torture or training. A punishment is clearly something the sub doesn't like when training and torture even if they involve pain have the goal to create pleasure end enjoyment.
Discipline involve positive reinforcement too, so when the sub behave he
got rewards. Also after each session it's important to tell your sub if he was a good boy and do "after care" like cuddles or hugging.
In BDSM there is rules, they are clearly defined between the Master and
the sub. If the sub breaks the rules there is consequences. Once again,
the level of the rules depends off the level of experience the sub have.
Total power exchange is the last level of obedience a slave can reach.
The goal of discipline is to teach the sub when he has made a mistake so
he can become a better one in the future. The punishment is always linked with the mistake and proportionate. I like to offer the option to the sub to choose between 2 or 3 different punishment. I also love to get 2 subs at the same time and to challenge them. Per example one is giving some spanking to the other one but I think it's not intense
enough the sub will be punished, but as the roles will be reverse after, he will take care to not be too mean too.