@rules is not a rule engine! @rules is a rules API that uses annotations to define rules where they should be defined when programming an application, on entity classes, in this case on POJO's. This way programmers can define the POJO's rules by defining the rules for each fields using annotations.
This is a straightforward API that uses just Java, and not some other DSL to define de rules.
This API goal is to solve most of the business rules needs for a regular Java application, to keep it simple, and hopefully help you write more manageable code.