A functional requisite is a major task a society must satisfy in order to survive. There are five functional requisites.
Without this first requisite, the population would diminish through attrition and death. The society must perpetuate itself in sheer numbers.
New members must be socialized into the society, or else it will descend into chaos. Socialization can be achieved through religion, education and other institutions.
Goods and services must be generated, and disseminated through economic institutions.
There must be a protection against internal and external attack. Police to patrol the streets and military to defend against invasion fulfill this requisite.
A sense of purpose brings people under the yoke of society, even if perpetuating the society requires personal restraint and even sacrifice. Without purpose, the society is destroyed by individual self-interest.