I.. I don't really know
The SPOT is "SPam and Off Topic" threads, but the problem with calling an entire thread offtopic is that the thread will have a topic unless it's spam. However if it was just called the Spam forum, that would propagate the notion that if such a place exists, then it's acceptable. In the OMGWTFBBQ Debates, I think the general consensus was that it's not really acceptable to spam for spam's sake.
My personal guidelines for this are as such: If the thread creator makes his thread in the SPOT, then they think it's offtopic enough to be there so that's fine. If someone creates a thread elsewhere that I think belongs in the SPOT, I'll move it. The criteria for such is not set in stone by any stretch of the imagination, but usually I mostly move those single-purpose threads such as "I brok my fone" or the like.
So I guess the cutoff line for me is thread quality, in a more direct manner of speaking. Typically, something that ends up in the SPOT has little chance of getting moved out unless such action is requested.