I often use it with the layer B VCA env amount. So it's just a Automated volume of Layer B. (and sometimes I then put env3 as the destanation of modwheel, so i can dial that in) THese things are super fun when you put a gated seqeuncer/arp on layer B)
I also use it as a glide mode (env3 destination Osc Freq ALL), but It glides with the same value everytime, sometimes from beneath sometimes from above.