JSYSTEM is Nintendo's in-house framework they use to make their games, including games from the GameCube and Wii era, and probably other consoles such as the 3DS. It probably has support for the SZS (aka Yaz0) compression format, and BMG message files. JSys is used in most first-party developed games.
The first game which JSYSTEM was used in development of was Luigi's Mansion, for the GameCube. JSYSTEM was still an early development project by workers of Nintendo EAD at the time. It was done being worked on when the DS came out. It was used for most of the first-party Nintendo games until it started being used alongside NintendoWare during the Wii era and eventually replaced by NintendoWare.
Games that are confirmed to have used JSYSTEM in its development include Luigi's Mansion, Super Mario Galaxy, Super Mario Galaxy 2, Mario Kart Wii, The Legend of Zelda: The Wind Waker, The Legend of Zelda: Twilight Princess, and The Legend of Zelda: Skyward Sword.
Known JSYSTEM libraries