Detergent.
I believe that if you put liquid (yuk!) detergent into the fabric softener cup it would dispense the detergent into the main wash after the prewash, provided there was a spin between them. Doing this would not, however, allow you to have fabric softener automatically dispensed.
Oh how I want an A700S or A702S,
Dave
And thanks for the explanation regarding the differences between the 2 washers.