Custom requests are defined for application-specific operations that do not fall under the normal pre-defined J2EE operations. Use custom requests to track specific application operations as separate requests. For example, if the application is completing some well-defined processing like parsing documents, or CPU-intensive numeric calculation, it can be useful to track this operation as a separate request.
When you use custom requests, you can set the monitoring level to L2 rather than L3. L3 generally applies to a large set of methods, typically every application method, as a result L3 monitoring processor usage is high. Custom requests are defined for specific application methods, and the processor usage is much lower. Also, custom request definitions are not limited to application methods, you can define specific system methods as custom requests.