A menu setting allocates channels 7 & 8 either as extra individual mono tracks, or for the stereo mix of tracks 1 thru 6, as I recall.
(edit) oops - I was looking at the manual just now and it seems I was wrong. 7 & 8 are a stereo pair (only?), and can be set for either a/ a stereo mix of channels 1 thru 6, or b/ as two additional tracks (digital input, stereo pair).
Interestingly, channels 5 & 6 can be set to use either their 1/4" jacks or the digital inputs, instead of 7 & 8 using the digital ins (DR-680 manual, pp 27-28). So you can have 4 analog in, 2 digital in, and the stereo mix on 7 & 8.
Must read more manual...
Dave