I think recognizing it's a different problem than billing and have it as a different entity is important. And of course that knowledge needs to be materialized

I think recognizing it's a different problem than billing and have it as a different entity is important. And of course that knowledge needs to be materialized into a technical block. Now, I'm totally with you that calling that an "engine" is misleading for 99.9% of businesses. In most cases that's a configuration file, or a business class if they're dealing with more user behavior check, calculations or conversions.