Entities encoded in UTF-16 MUST and entities encoded in UTF-8 MAY begin with the Byte Order Mark described by Annex H of [ISO/IEC 10646:2000], section 2.4 of [Unicode], and section 2.7 of [Unicode3] (the ZERO WIDTH NO-BREAK SPACE character, #xFEFF). This is an encoding signature, not part of either the markup or the character data of the XML document. XML processors MUST be able to use this character to differentiate between UTF-8 and UTF-16 encoded documents.
 
Although an XML processor is required to read only entities in the UTF-8 and UTF-16 encodings, it is recognized that other encodings are used around the world, and it may be desired for XML processors to read entities that use them. In the absence of external character encoding information (such as MIME headers), parsed entities which are stored in an encoding other than UTF-8 or UTF-16 MUST begin with a text declaration (see 4.3.1 The Text Declaration) containing an encoding declaration: