As mentioned earlier, XML is only used when writing a DTD. This is not strictly true. There is certain XML syntax that you will want to be able to use within your documents. For example, comments can be included in your document, and will be ignored by the parser. Comments are entered using XML syntax. Other uses for XML syntax in your document will be shown later too.
Obviously, you need some way of indicating to the XML processor that the following content is not elements within the document, but is XML that the parser should act upon.
These sections are marked by <! ... > in your document. Everything between these delimiters is XML syntax as you might find within a DTD.
As you may just have realized, the DOCTYPE declaration is an example of XML syntax that you need to include in your document…