Hey GB, would have been nice if they explained what they intended with wet/buff. So buffers are generally used to buffer voltage or current between one device and another (so a part of a audio circuit or 2 separate audio devices (in this case it's 2 audio devices)). (Kind of lets 2 things (that may normally not connect optimally (e.g. voltage/impedance/current miss match)) connect, i.e. buffers between them). e.g. high impedance to low impedance (http://en.wikipedia.org/wiki/Buffer_amplifier).
So I'd proffer the wet/buff is to plug into things like FX returns, mixer inputs etc; where as the wet/dry is designed for guitar input, guit amp, MP1 input etc. The manual doesn't give much detail but that's my take so far.
Cheers R