SZS (File Extension)
SZS is a file extension used in many games, including Mario Kart Wii. All of the tracks and battle courses are stored as .szs. The .szs extension is very misleading because many different file formats use this extension. Three examples are U8, SARC, an archive format used on the 3DS and Wii U, and BFRES, an update for the Wii U of the BRRES format. These are three different file formats and their content and use are also different. This article describes the relation between these files and why they are stored as .szs files.
In Mario Kart Wii
In Mario Kart Wii SZS is known as an archive file format. This format is actually known as U8 (because of its magic) and information about it can be found here. This means that it's actually U8 file and not a SZS file. This can be very misleading. One example where a .szs file is not a U8 file is Filesystem/Boot/Strap/*/**.szs where "*" is a release location and "**" is a language. In these folders there are .szs files stored but they are not U8 files. They are actually Yaz0 compressed BRRES files. This means that a file is not an archive file (U8) just because it's stored as .szs.
In Mario Kart 8
In Mario Kart 8 there are two different file formats that use the file extension .szs. They are SARC and BFRES. SARC is a archive formats, just like U8, and BFRES is a model resource format. When a SARC is uncompressed it is usually stored as .arc and when BFRES is not compressed it is stored as .bfres. Yaz0 compressed SARC files are stored as .szs and Yaz0 compressed BFRES files are also stored as .szs. This means that the file extension .szs indicates that a file is compressed using Yaz0 algorithm.
SZS is only a file extension, not a file format. It is short for Seven Zip Stream (or something similar) and indicates that a file is compressed using Yaz0, Nintendo's favorite compression that has been used since GameCube. No files should be known as SZS files. They should be known as the file content's file format. SZS files should be decompressed and then analyzed for better knowledge of the actual file format.
Since SZS is only a file extension and not a file format no generic tools exist. Many tools support Yaz0 compression though such as CTools, Every File Explorer and Wiimms SZS Tools. Every File Explorer and Wiimms SZS Tools can decompress any file so that any file can be easily analyzed.