How Does an RFID Reader Differentiate from Among a Large Group of Tags?

Tags and readers communicate using an air-interface protocol. Think of it as the language of an RFID system. There are international standards for air-interface protocols. This is like establishing a common language for a meeting of diplomats from a dozen different countries. If tags and readers from different manufacturers all use the same air-interface protocol, then they can communicate with each other, just as diplomats could all communicate using, say, English at the meeting.