Sub-category discoverability

I just spent a confusing moment looking for the Collection Development category. Or rather sub-category of the Project Discussions category. It doesn’t show up in the left side-bar. After clicking All categories in the left side-bar, it doesn’t show up in the, er, “middle side-bar”. If you didn’t already know it existed, how would you find it?

Until now, I didn’t know the forum even had sub-categories. What other sub-categories exist, and how would one discover them without prior knowledge?

D’oh! It does show up in the “middle side-bar”.
I’mma go clean my glasses and try again.

Sorry!