With reel-to-reel, some systems utilized 25 Hz with the start of the tone starting the next event, and the end of the tone stopping the tape deck.
Some used 25 Hz on left or right channel to get multiple types of closures. 25 Hz left only would trigger one thing, 25 Hz right only triggers something else, and 25 Hz both channels same time triggers yet another. In some cases that would be expanded to use two different frequencies, like 20 and 30, along with the combinations noted.
Adobe Audition has some great tools. If you need to deadroll something into a system which starts recording on audio, you can generate a 20 Hz tone at a low level, add silence, and voila, you've got something that will deadroll and work with automation. But the greatest thing since sliced bread on some of these digital editors is the ability to stretch and shrink without pitch change. You can trim every commercial to exact length... what did we ever do before this stuff was invented (other than be more precise with our original production)?