Service classification is an important step in realizing and creating an effective SOA implementation. .A Good classification system can simplify the task for finding services, promoting re usability and service composition. It is easier to search for services using query like - "find all services serving home loan". In the above query, home loan is a entity, which is part of your classification system.
Classifying the services can describe the capability of services -like "the transaction time for the service ", " security requirements for the service" and other service capabilities , based on your requirements.
In order to create entity and relationships, you can use Ontology Web language and than create taxonomies. Taxonomy is a system of hierarchical types that can be used to describe entities. The types are expressed in a class and subclass system. For instance in the above example - Loan is type of entity, which has sub classes Home Loan and Auto Loan.
The classification system would be typically created in your Service Registry. Service Registry like IBM WebSphere Service Registry and Repository provides you to ability to create a classification system for services either by loading an OWL file or using the user interface which abstracts out the OWL format from the user.
For creating OWL files you can use - IBM Integrated Ontology Development Toolkit, which provides a tooling environment to create OWL or an open source toolkit like Protege.