Rebcode is a dialect for creating very high performance code which can be useful for heavy computations and for creating algorithms.
In essence, Rebcode is an assembly language within REBOL. Rebcode should only be used when it's necessary, since it's based on performance, not ease of use, and is tedious to work with for longer scripts.
Also since Rebcode is designed for performance, it does not check for mistakes, and mistakes in Rebcode can crash REBOL, just like any other assembly language.
Rebcode is currently pre-alpha and is not finished. This section of the book will not be completed until we have a stable version of Rebcode which won't change too much or the release of a public release or late beta.