Both SI and CSI are traits that increase the frequency of fertilization of ovules by outcross pollen, as opposed to self-pollen.
Robert Bowman outlined the distinction when he posited that cryptic SI allows for full seed set via self-pollination when outcross pollen is limited or absent.
The contemporary understanding of this breeding system, which involves self-pollen discrimination, outlines the "best-of-both-worlds" hypothesis that was described by Bowman in 1987;[2] and later refined and given a name by Becerra and Lloyd in 1992.
Bateman in 1956 as a weak incompatibility system that results in a significantly higher proportion of seeds set by outcross pollen within an individual as opposed to self-pollen, when both types are present on the stigma in equal amounts.
There are multiple known mechanisms through which CSI acts but it is commonly defined as a form of parental selection that occurs post-pollination.