NDX = capitalization-weighted index (i.e. the higher marker cap companies in the index will be a greater % of the index) [1]

XNDX = a total return index tracks the price + reinvests all dividends back in to the index [2]

[1] https://www.investopedia.com/terms/n/nasdaqcompositeindex.as...

[2] https://www.investopedia.com/terms/t/total_return_index.asp