Order 7110.65 3-1-1 directs controllers to "Provide airport traffic control service
based only upon observed or known traffic." The FAA defines "known traffic" as "aircraft whose altitude, position, and intentions are known to ATC." Only after assigning a squawk, requesting an ident, establishing visual contact from a control tower or otherwise confirming an aircraft's identity and location, is the aircraft considered "known" and we can provide separation services.
Courtesy of 7110.65 3-1-1, I'm required to "Provide airport traffic control service
based only upon observed or known traffic." The planes I'm talking to are "known" traffic.