From Forum Nokia Wiki

Content Handler API (CHAPI), JSR 211, allows MIDlets to be integrated with device environment, opening and handling documents in the phone using a MIME-type or schema. CHAPI provides the abilities for browsers and native applications in the phone, as well as other Java ME applications, to invoke other Java ME applications which dynamically extend the media types and capabilities supported by the device's application environment.

For example, you can make an specific DOC format reader for the phone with Java ME. When the user selects a DOC file, our Java ME applications will fired to manage it.