I think these options refer to whether the selected rejection algorithm is applied for high and low pixels. These tooltips makes it sound like all values above the median will be rejected, no matter what algorithm is used. Some of the more sophisticated rejection algorithms will remove outliers, but not all pixels above the median.
https://github.com/PixInsight/PCL/blob/0bb6378c59dc2f60be4d71715c09393f5a5a0046/src/modules/processes/ImageIntegration/ImageIntegrationInterface.cpp#L1773-L1789
I think these options refer to whether the selected rejection algorithm is applied for high and low pixels. These tooltips makes it sound like all values above the median will be rejected, no matter what algorithm is used. Some of the more sophisticated rejection algorithms will remove outliers, but not all pixels above the median.