I think a poly book is for showing your learning progress, posting Work In Progress's, and asking for help in your learning.
Not everything you post in a poly book would be posted in the gallery.
A poly book is kind of like a portfolio that has all your questions and WIPs in there too. As you get better, you can look back into your poly book and see how far you've come and how you got there. It's kind of like your art history, still being written.
That is so awesome, thanks for answering that. I feel a bit disappointed though since didn't start one a year ago when I joined and the only history of my progress is trying to remember what went before what and looking at modified dates. But maybe I will start one now. Still a ways to go for me I think.
a sketch book is for 2D drawings and a poly book is for 3D (polygon) models and as already stated WIP that you don't post in gallery...