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https://www.w3.org/TR/xpath20/
Before an expression can be processed, its input data must be represented as an XDM instance. This process occurs outside the domain of XPath, which is why Figure 1 represents it in the external processing domain. Here are some steps by which an XML document might be converted to an XDM instance:
An atomic value is a value in the value space of an atomic type, as defined in [XML Schema].
[Definition: The term XDM instance is used, synonymously with the term value, to denote an unconstrained sequence of nodes and/or atomic values in the data model.]
Figure 1: Processing Model Overview
Figure 1 provides a schematic overview of the processing steps that are discussed in detail below. Some of these steps are completely outside the domain of XPath; in Figure 1, these are depicted outside the line that represents the boundaries of the language, an area labeled external processing. The external processing domain includes generation of an XDM instance that represents the data to be queried (see 2.2.1 Data Model Generation), schema import processing (see 2.2.2 Schema Import Processing) and serialization (see 2.2.4 Serialization). The area inside the boundaries of the language is known as the XPath processing domain, which includes the static analysis and dynamic evaluation phases (see 2.2.3 Expression Processing). Consistency constraints on the XPath processing domain are defined in 2.2.5 Consistency Constraints.
https://www.mimuw.edu.pl/~czarnik/zajecia/xml10/07-en-handout.pdf
XML and Modern Techniques of Content Management – 2010/11
