You can choose between four so-called chess-engines. By "engine" we mean that particular piece of software that makes Rebel calculate a move.
You can choose between the engine of Rebel Century (standard loaded), Rebel 9.0, Rebel 8.0 and the engines that simulate Rebel 7.0 and Rebel Decade. The ELO-rating that this engine is supposed to have on your particular PC will be changed automatically.