Tone

The signal a locating transmitter puts on a line, named for the audible note the receiver produces when it picks it up. "Putting a tone on the line" is the everyday phrase for energizing a target so it can be traced.

Transmitters offer a range of frequencies, and the choice matters. Lower frequencies stay on the target line better and bleed over less, but need a good connection and travel less readily onto poor conductors. Higher frequencies couple onto almost anything, which is useful when the connection is marginal and a liability in a congested corridor because they jump to neighbouring metal just as easily.