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||Unknown, possibly "Revolution FaceLib"|
|NAND||Related to the filesystem and IOS?|
|SC||Responsible for managing /shared2/sys/SYSCONF.|
|DVD||Related to reading Wii discs? Or maybe native DVD support?|
|KPAD||Possibly USB keyboard?|
|HBM||Possibly short for "Home Button Menu;" probably the HOME Menu|
|IPC||Handles communication with IOS|