%x.class;
X - Generated Text/Punctuation Class Elements
Class containing a single element that will hold generated punctuation or other generatable text, for example, the commas or semicolons between keywords
Model
<!ENTITY % x.class "x" >
<!ENTITY % x.class "x" >
Journal Archiving and Interchange Tag Library NISO JATS Version 1.1 (ANSI/NISO Z39.96-2015)
Version of December 2015