TAX AND TAPS

Dear sd experts

i have one doubt plz let me know ... i know what is the purpose of Materiel determination process and use of this ,, generally we use TAN Item category for this

And some time We can use TAX ( MAIN ITEM CAT) TAPS ( Sub item cate )

if we use TAX main item will be priced .. and if we use TAPS sub item will be priced

it little bit matches BOM alos but i want to know which business process we use exactly TAX AND TAPS