If band disagreement is raised then after discussion with manager what are the next steps??

Resign

Nothing will happen, they won’t listen you and justify themselves that they are correct and you are wrong. Basically you have not done anything. They will ask you that they have compared with same grade not didn’t find you suitable. Then if you ask for data on what basis that comparison done they won’t tell you. You just ask same thing how can u compare with out data. That data point will tell where I’m lacking. They don’t tell anything. All managers has there own good book resources and shoe licking resources and managers.