[boost] [msm]how to distinguish region ids?

Hello, On Boost.Msm 1.46.1, When I use explicit entries and entry pseudo states, I have to provide a template argument "region id". http://www.boost.org/doc/libs/1_46_1/libs/msm/doc/HTML/ch03s02.html#d0e930 But I couldn't find documents about numbering policy of region id. I also check the example below. http://www.boost.org/doc/libs/1_46_1/libs/msm/doc/HTML/examples/DirectEntryT... The example passes the region id 0 and 1. I'd like to know how to decide which region is 0(or1). Am I missing something? Thanks, Takatoshi