1.4.2 Tags (field numbers)

A tag (or field number) is a small integer that is present in every wire-stream primitive. The tag is the only means that the interpreter has to synchronize the wire-stream with its template. Tags are user defined for each term in each message of the wire-stream. The protobuf specification requires that each field within a message has a unique field number; the protobuf compiler (protoc) will produce an error if a field number is reused (field numbers are unique only within a message; an embedded message can use the same field numbers without ambigituity).