Rules are usually set and agreed upon by the dominant and the submissive before the beginning of any BDSM scene and/or situation, and can also be used to help make the sub feel inferior, or for "training" a novice sub, though such is not always the case and rules can be set by the dominant for reasons both pertaining to the scene (such as fear play) and/or reasons outside of the constructed world of BDSM.
Punishment itself can be physical (e.g. whipping or piercing) or psychological (e.g. erotic humiliation through public nudity or golden showers) or a combination of both (e.g. through predicament bondage).
[citation needed] The goal of discipline is to "teach" (in the context of the BDSM scene, and only as such to make the sub feel inferior by playing as if they are being taught "how to behave" like a child or animal, and such is pretended, as BDSM is sexual play and does not pertain or adhere to reality) the sub how they should behave as well as the consequences that may arise as a result of breaking the rules of behavior.
For a major indiscipline by a male sub, the choice may be the instrument type to be used (e.g. whip or cane) and the body part to be punished (e.g. back or buttocks) with a number of hard strokes.
Punishment should also not be confused with BDSM training which may involve giving pain simply in order to increase the endurance limit of the sub.