WBP means "Water Boil Proof", which basically means you can use it externally.
Some words of caution though. I was given several sheets of shuttering ply a few years ago (cheap looking stuff, with the edges of the sheets painted blue-green), and I used bits of it for a number of different projects.
Most of it lasted less than a year before it started to delaminate, and had to be replaced entirely within two years. You pays your money and you takes your choice!
BTW, we built a duck house from tongue and groove weatherboarding three years ago, and it's still going strong. Perhaps not as good for redmite prevention, but I thought I'd mention it, as I don't recall it being that expensive.
Edit: ClydesdaleClopper - where's the best place to buy Stockboard / Stokboard? I've never actually seen it for sale anywhere as sheets?