"may operate as a non-diversified fund ... to the approximate extent the index is non-diversified" - what does it more specifically mean?

Letter from Fidelity the other day mentioned the followings:

"Effective November 10, 2025, the “Principal Investment Strategies” and “Principal Investment Risks” sections of the prospectus of each index fund shown in the table below was modified to indicate that the fund ***may operate as a non-diversified fund***, as defined under the Investment Company Act of 1940, ***to the approximate extent the index is non-diversified***.

A non-diversified fund may invest a greater portion of its assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified fund.

Shareholder approval will not be sought when a fund crosses from diversified to nondiversified status due to changes in the relative market capitalization or index weighting of one or more index constituents.

| Fund | Index |
|:---|:---|
| Fidelity 500 Index Fund | S&P 500 Index |
| Fidelity SAI U.S. Large Cap Index Fund | S&P 500 Index |
| VIP Index 500 Portfolio | S&P 500 Index |
| Fidelity Total Market Index Fund | Dow Jones U.S. Total Stock Market Index |
| Series Total Market Index Fund | Dow Jones U.S. Total Stock Market Index | "