What is the Superko rule in Go? Is it used in most scoring/online systems, or only in certain ones?
There are two main Superko rule types, and, as long as the ruleset used isn’t Japanese, one of them should be in place.
Namely they are Positional and Situational.
Positional superko is used in the Chinese Rules, and prohibits the repetition of a position or board state, regardless of whose turn it is.
Situational superko is used in the AGA and BGA (American and British Go Association) rules, and prohibits the repetition of a position or board state, with the same player to play.
The variant Natural situational superko used by the BGA only forbids repetition by the player who created the position.
For more information on superko take a look at Sensei’s Library