What's it imitate? Darned if I know. The IOBO seems to work equally well as a Caddis Fly, a Mayfly, a Terrestrial or a Cranefly. Lets just say it is a highly effective attractor.
Even size doesn't seem to be very important. For several years I only fished size #16. I do use different sizes now. In fact I've found recently, I'm fishing smaller IOBOs. Sizes #22 and #24 work great in Trico season and #20s and #22s do extremely well in BWO hatches.
Although, I usually think of the IOBO Humpy as a Caddis imitation, I can easily infer that the trout might see it as a beetle or other terrestrial.
You are free to make up whatever story you wish about what it matches. After all, tyers and anglers have been doing it forever.