Most characters in an XML document can be expressed as regular Unicode™ codes. Characters not in the standard Unicode™ tables have traditionally been expressed as explicit font calls (non-portable), created as inline graphics, or defined in one of the Unicode™ private-use areas where users may define their own characters. This element is used when there is known to be no font available to render a private-use character. The <glyph-data> element provides information on the actual glyph that is associated with the private-use character.