glXAllocateMemoryNV parameters (last three)

I am looking for a man page or explaination of the parameters that get passed into glXAllocatMemoryNV. I am particularly interested in the last three values. What values for read and write frequency get me which kinds of memory (AGP, Video, ??)? What does the priority mean? Why would it change as in the VAR examples where it shifts from .5 to 1. in reallocations?