H = VL_AIBHIST(PARENTS, DATA) computes the histogram of the data points DATA on the VL_AIB tree defined by PARENTS. Each element of DATA indexes one of the leaves of the VL_AIB tree.
H = VL_AIBHIST(PARENTS, DATA, 'HIST') treats DATA as an histograms. In this case each compoment of DATA is the number of occurences of the VL_AIB leaves corresponding to that component.
H has the same dimension of parents and counts how many data points are descendent of the corresponding node of the VL_AIB tree.