--ftop n features in hclust2.py

Hi Guys! If anyone of you has used metaphlan3 for the shotgun metagenomic datasets and made the heatmaps using the inbuilt script called hclust2.py then I have a question. There is a flag —ftop n (n is the integer which you want to select as top features/taxa), I want to know how do you select the top n features based on the percentile rank of the relative abundance values of the particular taxa/features among all the samples. Please explain if anyone knows it.

Thanks
Saraswati Awasthi