And as you can see, the W3C are not a registered owner either. There are several keywords that indicate the type of information in the file. Any description you want to supply for the contents of this file. This may include version numbers or any short text that is meaningful to you and unique for the SGML system. This is an ISO two-character code that identifies the native language for the file.
EN is used for English.
In order to do this it can use a catalog file. The SGML processor would know to look up the DTD from strict.dtd in the 4.0 subdirectory of whichever directory held the catalog file that contained that line. In order to locate a catalog file, your SGML processor will need to know where to look. Many of them feature command line parameters for specifying the path to one or more catalogs. Using FPIs is preferred for reasons of portability.