The phrase "official subtitle" refers to the text displayed below the title of a book, movie, or any other media. It provides additional information about the content or context of the main title.
Sentences with «official subtitle»
That's the official subtitle for Animal Crossing 3DS in North America. (animalcrossingworld.com)
And yes, «A Star Wars Story» is clearly the official subtitle for these spinoff properties. (space.ca)