Sony DVW-250 - Manual (Page 43)

In the Digital Betacam format, in addition to the digital
audio channels recorded on the diagonal tracks, there
is also a longitudinal track, referred to as the cue track,
which can record a single analog audio channel.
The cue track can be used when adding an audio
channel to an already recorded tape.
Connections and settings for audio
dubbing
Connect the microphone or other audio source to the
AUDIO IN connectors. In audio dubbing, the input to
channel 4 is recorded on the cue track.
Connections and settings for audio dubbing
Chapter 4 Recording 4-15