An IDREF-type attribute that points to the @id attribute of the <glyph-data> character inside a <private-char> element. The idea is to use the full <glyph-data> with its name and bitmap once, then point to an existing character instead of defining the entire <glyph-data> element repeatedly.
Once a private character has been declared using a <glyph-data> element, the character can be reused by using this attribute on a <glyph-ref> element to point to the existing full <glyph-data> element (bitmap included).
|An identifier (IDREF)||The pointing uses the ID/IDREF mechanism, using the @glyph-data attribute of the <glyph-ref> element to point to the @id attribute of an existing <glyph-data> element.|
|Restriction: This attribute may be specified if the element is used.|