及其 is a formal conjunction used primarily in written Chinese to connect two nouns, where the second noun is often a possession, component, or sub-unit of the first. It combines the conjunction 及 ('and') with the possessive pronoun 其 ('its/their/his/her'). Use it in official reports, academic texts, or legal documents — not in casual conversation. Unlike 和, 及其 explicitly indicates a possessive or subordinate relationship between the two items.