I recently bought a used Fostex Model 20 1/4" 2-track reel-to-reel deck. On the rear with the stereo in/outs, it also has 1 RCA "CUE IN" and 1 RCA "CUE OUT". Also, on thefront face panel, it has a knob labeled "CUE CAL"; my question is what is "CUE" for?
I could be mistaken.....but I believe the "cue" was used as a center track for recording a signal pulse. This was used in conjunction with a Kodak Carousel projector. Music and dialog could thereby be "Synced" with a slide presentation.
The cue signal would advance the projector to the next slide.

Dom Franco
Now that you mention that and the more I think about it, I think you're correct about it having to do with syncronization. On the front face plate, it have a "CUE IN" knob (not a "CUE CAL" knob); my mistake. It also has a meter labeled "CUE" along with the "IN" and "OUT" and seperate "SYNC" buttons for the "L" and "R".