Where are options to TTree::Process() documented?

I am looking at the Reference Guide for ROOT 4.04/02 (latest production). Where can I find documentation on what the allowed values for Option_t are in the TTree::Process() function, and what each of those allowed values mean?

This is actually a much more general question, as I have found it systematically difficult to determine what options to use for any given function.

The only thing TTree::Process does with the option is to pass it on to the TSelector (see $ROOTSYS/tutorials/h1analysis.* for an example)