For example, XML permits whitespace to occur at various points within start-tags, and attributes to be specified in any order.
Such differences are seldom if ever used to convey meaning, and so these forms are generally considered equivalent: In converting an arbitrary XML document to Canonical XML, attributes are encoded in a normative order (alphabetical by name), and with normative spacing and quoting (though with all namespace declarations placed ahead of regular attributes, and namespaced attributes sorted by namespace rather than prefix or qualified name).
Canonical XML specifies a number of other details, some of which are: According to the W3C, if two XML documents have the same canonical form, then the two documents are logically equivalent within the given application context (except for limitations regarding a few unusual cases).
For example, a steganography system could conceal information in an XML document by varying whitespace, attribute quoting and order, the use of hexadecimal vs. decimal numeric character references, and so on.
On the other hand, XML files that differ in their use of upper- vs. lower-case, or that use archaic versus modern spelling, and so on, might be considered equivalent for certain purposes.