R2ML is a comprehensive and user-friendly XML rule format that allows
- interchanging rules between different systems and tools,
- enriching ontologies by rules,
- connecting your rule system with (our) R2ML-based tools for visualization, verbalization, verification and validation.
R2ML is comprehensive in the sense that it integrates
- the Object Constraint Language (OCL) - a standard used in information systems engineering and software engineering,
- the Semantic Web Rule Language (SWRL) - a proposal to extend the Semantic Web ontology language OWL by adding implication axioms,
- the Rule Markup Language (RuleML) - a proposal based on Datalog/wikipedia:Prolog, and it includes four rule categories: derivation rules, production rules, integrity rules and ECA/reaction rules.
R2ML is a usable language in the sense that it allows structure-preserving markup and does not force users to translate their rule expressions into a different language paradigm such as having to transform a derivation rule into a FOL axiom, an ECA rule into a production rule, a function into a predicate, or a typed atom into an untyped atom.
Notice that R2ML, like OCL and OWL/SWRL, provides a rich syntax for expressing rules supporting conceptual distinctions, e.g. between different types of terms and different types of atoms, which are not present in standard predicate logic. However, you don't have to be familiar with all of R2ML's language elements in order to use it productively.