RegInit.ini is the default system registry for a particular CE build. This registry, in conjuction with ADSLOAD.REG, defines the system registry when the CE system boots.
If the Persistent Registry feature is enabled, the loader reads the complete registry from a partion of onboard flash and replaces the entire system registry.
The system registry defines what drivers are available to the system at boot, as well as many other operational parameters. Comments in reginit.ini offer insight into how the system operates.