The four “score sets” are computed to account for configurations in which certain rules are never used. For example, score sets 0 and 1 assume that Bayes has been disabled, while sets 2 and 3 are used when Bayes is enabled. Sets 0 and 2 are used when network tests have been disabled, whereas sets 1 and 3 are used when network tests are enabled.
http://www.renaissoft.com/pipermail/maia-users/2005-March/004054.html
