UL913 is a certification for Product safety with intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1, Hazardous (Classified) Locations is a unique situation.
Currently there are two active editions of the standard: 5th and 7th.  (The 6th Edition was withdrawn in April 2008 in the sense that the standards writing body did not think any further certifications should be issued to it.)
While both cover the fundamentals of intrinsic safety, there are significant difference between the two; the 7th edition is harmonized with the international requirements of the IEC 60079 series.  This results in a more stringent edition of the standard that employs enhanced requirements.