Needs and Wants
Needs are desires of a submissive (or dominant) which cannot go unmet over time without harm. This is in contrast to wants which can go unmet without harm. These can also exist in non-binary relationships as well.
In most M/s contracts, it is the d-type's responsibility to ensure that needs are addressed, although the practicalities of achieving this may be delegated to the s-type in question or a third party. Different needs have different levels of urgency and frequency, and the urgency or lack of urgency of a need reflects this. Some needs, such as oxygen, are not at all elastic and a d-type has little scope in prioritizing them; but others, such as leading a purposeful life and growing as an individual, are very elastic and frequently put aside for a time in favor of more important or practical tasks.
In cases where M/s or other power exchange is not already negotiated for and present it is up to the individual adult to determine their needs and wants as well as to prioritize them and negotiate for them.