MHEG-5 - (Multimedia and Hypermedia information coding Experts Group) - The MHEG-5 standard was originally developed a decade ago and offers basic interactive facilities through a declarative description language. A working group is developing or has developed a specification to enable the use of a broadband return path with MHEG-5. 
www.mheg.org/users/mheg/faq.htm MHEG-3 - MHEG regarding scripting.

MHEG-6 - (Multimedia and Hypermedia information coding Expert Group) - MHEG standards and specifications were developed to increase MHEG-5's performance and access to various external services. MHEG-6 is often associated with advanced MHEG interactivity.