Rules Engine is a library/NuGet package for abstracting business logic/rules/policies out of a system. It provides a simple way of giving you the ability to put your rules in a store outside the core logic of the system, thus ensuring that any change in rules don't affect the core system.
You need to store the rules based on the schema definition given and they can be stored in any store as deemed appropriate like Azure Blob Storage, Cosmos DB, Azure App Configuration, Entity Framework, SQL Servers, file systems etc. For RuleExpressionType LambdaExpression, the rule is written as a lambda expressions.