The type and URL are again character strings. The type has the same lexical constraints as the type of an element and the URL has no constraints. There is one exception to the rule that a document node must have a type. It only has an instruction i associated with it. Their meaning is explained below .
Comment node A comment node is similar to a processing instruction.
Data node Data nodes are also always leaf nodes. Data nodes cannot be empty, that is, their data characteristic contains at least one character. The data that is encoded in an XML document may or may not have embedded newlines. If it doesn't, and you still want to edit it by hand, the document may be difficult to handle with a simple text editor. XML therefore allows the insertion of newlines at certain places, which are not part of either the mark-up or the data.