what is a Persistent Constant?

Hello, when you define a constant you can use two things that are not very clear to me. Can someone explain? 1) CONST_PERSISTENT. How can a constant be persistent? The documentation says that the constant is not "forgotten" after the process stops. Well if the process stops, where is the constant stored? I couldn't find any place where the constant would be written to a file or some location. Then when then module is started again, all programs I have looked at register those same constants again, so what is the point of making them persistent? Something is not getting into my small brain, there. 2) REGISTER_MAIN_.. Those are supposed to remain in the symbol table. The only symbol table I see is the Zend constant symbol table. So if the process ends, then they're lost, and that's fine with me. But if a module registers a constant as MAIN, then gets unloaded, the constants stay, available for other modules I guess. So I understand REGISTER_MAIN (I think), although I don't really see the point, but i'm totally not clear about CONST_PERSISTENT. Anyone can explain? Thanks! Fab.