Multi asset fund means the the ETF or Mutual fund has multiple stocks from the different market cap or different asset class. Instead of having Small Cap ETF or Mid Cap ETF, people can go with the multicap and get the best of all the world's. It is best for most of the people because it is less volatile than the small cap and gives better returns than the large cap.

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The inflows in the Multi asset fund has increased too. For example, the Inflows in Multi Asset Fund is around 5K Crore whereas the small cap and mid cap is around 4K Crore. The best part of Multi asset fund is that it has to have 3 asset class. And all the asset class should have atleast 10% allocation. The asset class is like equity, debt, real estate, metals etc.
From the last 1 year the equity market is not that great and that's why people are moving towards the Multi cap because it seems to be giving better result than the equity. The thing is Multi Asset Fund is great when the market is down or in sideways but when the market is in bull run Multi asset doesn't give good returns.
So I am not saying that you should convert fully to the Multi asset fund. But people who don't want to see volatility in their investment, for them Multi asset fund is the best way to invest in market. They will get little less returns in the long run but at the same time the volatility will decrease a lot and they will sleep peacefully. But if you are someone who wants to get growth, then best is to have portfolio with large, mid, and small cap fund.