The top 10 holdings would be the top 10 holdings in the portfolio. The top 10 underlying holdings would be the top 10 stocks you have exposure to in the portfolio.
The simplest way to explain this is say you own 9 stocks and an S&P 500 ETF as your 10 largest holdings.
Your underlying holdings could be different, as it will take that S&P 500 ETF and look at the underlying holdings inside of that fund, and then list your top 10 companies you're exposed to.
Does that make sense? In our case, we don't have too too many ETFs in these portfolios, so for the most part they will be the same.