Stepweise step down post hoc test for Kruskal-wallis in SPSS


What are the criteria for computing the pairwise or the stepwise step-down comparison in SPSS after Kruskal-Wallis? I know that the pairwise comparison is more conservative than the stepwise comparison, but are there any other criteria to decide which post-hoc test to use? Does anybody know which method or test SPSS uses for the stepwise step-down comparison? I could't find any information about this on the IBM homepage. And is there a way to calculate p-values for the significant differences in the stepwise comparison as SPSS doesn't provide them?

Thank you very much in advance!:)