In a business buying centre situation, the people who sometimes initially request the product, who actually use the purchased product or service, and who may even assist in developing the product specifications are called

Users are the people who actually use the purchased product or service, initiate requests, and often help define specifications. They're distinct from influencers (who provide information), gatekeepers (who control information flow), deciders (who make the final choice), and buyers (who handle transaction mechanics). Users are on the front lines - they know what's needed from direct experience.