The Sectone is sometimes called the sec tone or sectone. People in the radio industry refer to it as the "segue" ( pron. "segway") point.
Its job is to inform DPS that its time to start playback for the next item in the sequence of events.
A song may have a duration of 3 mins, but may have to all intents and purposes, faded out to a very low level by 2:50. In which case, you should set the sectone to somewhere just before 2:50, so the transition to the next playlist item is smooth when the sequencers are on in the carts.