Report Engine API (= RE API) is a programming interface that is used to integrate BIRT functionality in your applications as POJOs so that you will be able to create and publish BIRT reports at runtime.
RE API allows:
Report Engine API allows embedding the engine within any Java/Java EE application. The BIRT Web Viewer uses this API to execute and display reports.
You can find more detailed information regarding API overview and its configuration and installation here.