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A Cogsys extension is a file that Cogsys loads and executes at run time which provides additional capabilities. You can think of an extension as part of the main Cogsys program that only gets used when it needs to be. Its mechanism is similar to plug-ins for Netscape Navigator.
Technically, an extension is a kind of dynamically linked library. Dynamically linked libraries are present in most operating systems: on Windows these files usually named with the `.DLL' extension; on Unix, they are called `libname.so'. Since Cogsys runs under Unix, the extensions will have a `libname.so' format, but the call in the testlist will be of the form `libname.cxr' for backward-compatibility.
What can an extension do? In theory, anything the real Cogsys does. In practice, the only extensions that have been created which the main program knows how to call are used for dynamically creating pictures. An extension routine that creates such images is called a picture generator.