A major flaw in a D/s relationship is consistency. You either stick with what You said or You fall apart. It is vital to the sub to know if something her Dom has said will change or will He stand by it. DOes He break promises, forget to do things? Those all flow together into trust. She has to trust things will not change at a drop of a hat and He will be there for her.
Another thing is what a Dom says. Words carry great weight and if You do not know the precise meaning of Your words Your intent is lost. Predominantly for me are the words may, shall, and will.
May- You have left it up to the sub to do it or not. It is their choice and if it is not done they will not be punished. Shall and Will- These have virually the same meaning. There is no room for choice, the task is mandatory. The sub has to do whatever it is.