The SDK (internally known as RVL_SDK) is a set of libraries linked with every official Broadway title, containing general functions, and implementing an OS. It is responsible for sending calls to IOS, and is responsible for other features such as the HOME Menu and Bluetooth/Wiimote code. Because of the Wii's architecture, it is impossible for Nintendo to update the SDK in every title, although it may be possible with homebrew.
SDK versions generally correspond to major IOS versions, however, this is not an exact correspondence; one notable example is the SDK version used in Red Steel, which has a HOME Menu with different sounds.
Many libraries can be identified by a string saying
<< RVL_SDK - NAME; this string also contains build info. However, libraries without that string can be identified by their functions, as all function names start with the library name.
|OS||Full name is Revolution OS; responsible for things like context switching|
|RFL||Full name is "Revolution FaceLib". For handling Miis.|
|NWC24||The WiiConnect24 API Library. Responsible for WiiConnect24 functions.|
|WPAD||The Low Level Wiimote Library.|
|GX||Responsible for the GX|
|NAND||Responsible for performing NAND operations, both the standard operations and the /dev/fs operations.|
|SC||Responsible for managing /shared2/sys/SYSCONF.|
|AI||The Audio Interface.|
|DVD||Responsible for reading files from Wii discs.|
|CNT||Content API Library. Responsible for reading the contents of a Wii Title.|
|CNTCACHE||Unknown. Interacts with /shared2/cntcache.txt before deleting it.|
|KPAD||The High Level Wiimote Library|
|HBM||Possibly short for "Home Button Menu;" The HOME Menu Library.|
|IPC||Handles communication with IOS|
|DEMO||Allows text to be printed on the screen. Used on discs such as the Wii Backup Disc.|