What are "requests that don't contain an application suffix"?

Programming Jakarta Struts by Chuck Cavaness wrote:

If you use only a single Struts configuration file, you will have only a single application. This is known as the default application. To make processing requests for the default application and the application modules simple and consistent, the default application is treated as just another application module. Therefore, any requests that don't contain an application suffix will be routed and handled by the default application.

This is a general note in the referenced text. I don't understand this note. I googled "application suffix", and I wasn't able to find a useful explanation. Can you help me?

Thanks,
Harry