Okay, I got it... So you open cab lab on your computer, load up to 8 IR's, then you can send that "mix" of all 8 IR's to scratchpad and see how it sounds. If you want to change things, you start tweaking the 8 IR's in Cab Lab (muting, mixing, etc.), then you send that new mix version to scratchpad and see how it sounds. So, you're not hearing the effect of the mixing/muting etc. until you send that latest IR mix to the scratchpad.
It sounds good, and I will use it for sure... just wish it was more real-time mixing/auditioning so you can hear the result immediately like some of those DAW plug-ins I've heard about...