This depends very much on the engine. You can find engines in either protocol that supports some form of difficulty setting, but the actual implementation varies. Some examples:TracySMiller wrote:OK, I've since learned that most of those engines are UCI, which the Strength setting in Arena has no effect on. Explains a lot. Apparently, only UCI2 engines are adjustable, if I'm not mistaken.
- Stockfish, Zurichess. Custom UCI parameter, from 0 to 20.
- Texel, Arasan, Cheng, Rybka, Ufim. UCI engines where strength can be specified by ELO. However, the actual ELO values are only approximations guessed by the engine author; Actual playing strength may vary from engine to engine.
- Crafty (multiple builds), Phalanx. Winboard engines where you can change strength and/or style using custom configuration files and/or startup parameters.
- Rodent. A special case; This engine is all about configurability. You can change all kinds of parameters to affect strength and style and create multiple distinct personalities, akin to what you can do with The King in Chessmaster.
I have played quite a bit with Stockfish. At level 0 it plays like a decent amateur, somewhere below 1000 ELO. Jumps in strength from one level to the next is quite consistently around 100 ELO.