It started out as a A W3C standard for what is and is not significant in an XML document, a description of what information an XML parser must provide to the client application:
An XML processor conforms to the XML Information Set if it provides all the core information items and all their core properties corresponding to that part of the document that the processor has actually read. For instance, attributes are core information items; therefore, an XML processor that does not report the existence of attributes, as well as their names and values (which are core properties of attributes), does not conform to the XML Information Set.
It became a list of definitions for particular information items:
Since the purpose of the Information Set is to provide a set of definitions, conformance is a property of specifications that use those definitions, rather than of implementations.
Specifications referring to the Infoset must:
Indicate the information items and properties that are needed to implement the specification. (This indirectly imposes conformance requirements on processors used to implement the specification.)
Specify how other information items and properties are treated (for example, they might be passed through unchanged).
Note any information required from an XML document that is not defined by the Infoset.
Note any difference in the use of terms defined by the Infoset (this should be avoided).