THX Surround EX: This designation for Dolby Digital EX or Dolby Digital 5.1 EX is based on an agreement between Dolby Labs and THX Lucasfilm that lasted until the end of 2001. This stated that the term "Surround EX", known from commercial cinema films and standing for the matrix decoding of the 6th channel (the rear centre), should only be used in Dolby's home cinema area in connection with the THX Logo could be marketed. So it always had to be called "THX Surround EX".
This proved to be a market obstacle and so the term "Dolby Digital EX" was created, which technically - at least as far as the 6-channel technology is concerned - has the same background as "THX Surround EX".