This version of SAX formally introduces a single way for optional parser features to be controlled from the application. All features work in the same way. The SAX standard includes some recommended features, and parser developers are encouraged to support them. They may also add their own features.
This technique has already been seen in the previous chapter (the Xerces parser implemented this technique in advance of the SAX 2.0 release), where this mechanism was used to tell the parser to validate documents against a DTD or XML Schema.
The feature name is a reference to a URL. This approach is taken for the same reason that the Namespaces standard uses URLs to avoid conflicts. Only complete URLs, including domain names, ...