Martin Fowler defines the Service Layer as “An Application’s boundary that defines the set of Services that the application can provide”.

While modeling applications where you are separating out Business Rules, or at least intend to do so at some point, it makes a lot of sense in separately modeling those calls that will need to be validated by your “Rules of Business“.

So, your top level layers now look like this.

• Presentation Layer
• Domain Model (Business Objects Layer)
• Enterprise Rules Layer
• Data Source Layer

The Rules Layer would provide the following:

• Validate the Business Information for correctness
• Hide the Business Rules Engine and Business Rule evaluation details from the rest of the application
• Expose only those Business Rule specific operations that are required.