The NISO STS Tag Sets have been designed with Unicode values as the basic representation of all characters, including what are frequently called in publishing “special characters”, for example: Greek letters, publishing marks such as section symbols and em-dashes, mathematical symbols such as roots and fractions, alphabets with diacritics, etc. The use of combining characters is supported and encouraged, as is the use of entities defined by the STIX project (http://www.stixfonts.org/). Unicode values in planes other than Plane 0 may be freely used. Use of private publisher entities and the Unicode Private Use Area is discouraged, but supported with the <private-char> element. With this element, a graphic image may be included, or a corresponding bitmap or alternative Unicode character may be included, using the <glyph-data> element.

Because of the potential for conflicts in assignments by different publishers, the NISO STS Tag Sets do not support assignment of values in the Unicode Private Use Area. Special characters defined by publishers as custom entities or in the Unicode Private Use Area are not typically handled by the NISO JATS Tag Sets. Publishers who have defined characters in the Private Use Area may:

• Remap those characters to existing Unicode values (using combining characters for special accented characters where appropriate), or
• Recreate each character as an image or as a bitmap using <glyph-data> or one of the other methods supported under the <private-char> element.