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.
Full definition
That's
the official subtitle for Animal Crossing 3DS in North America.
And yes, «A Star Wars Story» is clearly
the official subtitle for these spinoff properties.
However, this is typically not
an official subtitle.
Square Enix is yet to reveal
an official subtitle or full name for the game but we are told to expect more from Square Enix and Platinum this coming Autumn.
That's
the official subtitle for Animal Crossing 3DS in North America.
On May 20, 2008, Square Enix opened up the North American site featuring the three Dragon Quest DS remakes, acknowledging Realms of Reverie as
the official subtitle, as well as confirming a release in North America.