Readings Systems
A readings system is a romanization system implemented in such a way that instead of being limited to yielding just one romanization value for a character, can return every known reading for every single character.
This is a package-specific detail, but it allows you to do with the readings how you please. If you'd rather just get a reading, these systems still support the basic Process function.
If you're looking for the API Documentation for them, you can find it here.