how/when the ode45 solver is executed within a Simulink model as level-2 s function>>??

I'm trying to understand how/when the solver is executed within a Simulink model. Specifically, I have a rather complicated MATLAB script that uses ode45 that I need to convert to a level 2 m-file s-function for use within a Simulink model. It's not clear whether that code should go in the Outputs() function, the Derivates() function, etc. as I have yet to find any good documentation describing how all this stuff works.