Files with the .xsp-config extension are used to describe the controls that are available in the IBM Lotus Domino Designer tool. The information in the files is used in Designer to provide tooling support when editing the XPages XML files. For example, the xsp-config files list all the Controls that are present in the Controls palette, and if you select a control and view the Properties panel, the All Properties tab is populated by the lists of properties declared in the xsp-config files. The information in the xsp-config files is also used to build the .class file generated from each XPage XML file, so it needs to be accurate to prevent compile errors at design-time and to prevent ClassCastExceptions or other type related errors at runtime.
The xsp-config format is based on JSF's existing faces-config format, but the xsp-config files are only loaded at design-time in the Domino Designer tool and builders, rather than at runtime when the XPages are executing.