The systematic counting and categorization of populations by the state — which began in ancient Mesopotamia and Egypt as a tool of taxation and military conscription, became the instrument through which modern states know, administer, and govern their populations, and reveals in its categories (race, religion, language, occupation) how states define who belongs, who counts, and on what terms.
A census is not a neutral count. It is a political act. Every question on a census encodes a decision about what matters — what categories of person the state recognizes, what it needs to know to govern, and what it chooses not to count. The US census counted enslaved people as three-fifths of a person. The British Indian census created 'caste' as a rigid administrative category for a social system that had previously been more fluid. The Nazi census identified Jews. Who gets counted, in which category, determines who gets represented, taxed, conscripted, and protected.