The subset normalisation quantitaion can be used to normalise any quantitation to a specific subgroup of probes which would be expected not to change between your conditions. This could be something like a set of housekeeping genes, or a set of regions which you have experimentally profiled by some other means to determine that they are not changing and can act as suitable controls. The combined data from all of the probes in the chosen subset will be used as the basis for the correction.