What is the difference between node types and atomic types in XML
Sounds like you are not asking about XML itself, but about the XQuery/XPath/XSLT data model known as XDM.

In these languages there are two kinds of value: nodes, which basically means anything within an XML document, and atomic values like integers and strings which have no relationship to any XML document. So for example the query count(//employee) takes a set of nodes as input, and produces an atomic value (an integer) as output. An important operation is atomization, which extracts an atomic value from a node, for example the string that is the value of an attribute node.