One major difference to XML is that all SGML documents begin with an SGML Declaration, which can be seen as a kind of configuration file. Instructions in this section provide values for the characteristics and limitations described below.
The SGML declaration logically precedes the DTD. It may be present in the same physical data file as the DTD and document instance, but more typically it applies to many documents, and even to many document types, so is located in a separate data file.
There are default settings for all the features affected by the SGML Declaration. When the default settings are satisfactory, there is no need ...