When constructing a portfolio, fund managers in Europe operate under a specific set of diversification rules known as UCITS. The cornerstone is the "5/10/40" rule, a three-part constraint designed to prevent over-concentration in any single investment.

First, a UCITS fund can invest no more than 10% of its total Net Asset Value (NAV) in securities from a single issuer. This is a hard ceiling on any one company.

Second, there's a limit on how many of these larger positions you can hold. While you can hold up to 10% in one issuer, the sum of all your positions that are individually greater than 5% of your NAV cannot exceed 40% of the total NAV. This forces diversification across several medium-sized holdings, rather than concentrating the fund in a few large bets.

This structure prevents a fund from being defined by a small number of very large positions. If a manager holds four positions that are each 9% of NAV, their total is 36%. This is within the 40% limit. But they could not add another 6% position, as that would push the total to 42%. They would either have to reduce one of the existing large positions or keep the new one below 5%.

In the United States, diversified funds regulated under the Investment Company Act of 1940 follow a different guideline, commonly known as the "75/5/10" test or Section 5(b)(1) diversification test.

This rule splits the portfolio into two distinct parts: a diversified portion and a non-diversified portion.

For at least 75% of the fund's total assets, the manager must adhere to strict diversification. Within this 75% slice, no more than 5% of the fund's assets can be invested in a single issuer. Additionally, the fund cannot own more than 10% of the outstanding voting securities of any single issuer. This second clause prevents the fund from exerting significant control over a company's management.

This 25% basket provides significant flexibility. It allows managers to take high-conviction, concentrated bets on a few companies without violating the fund's diversified status. A manager could place 4.9% of the fund in nine different companies (totaling 44.1% and falling within the 75% bucket) and then place 25% in a single tenth company using the basket.

The differences become critical for global asset managers who offer "mirror" funds—a single investment strategy packaged as both a UCITS fund for European investors and a '40 Act fund for US investors. The manager must run the strategy in a way that complies with both sets of rules simultaneously.

In practice, this means the stricter of the two rules dictates portfolio construction. Because the UCITS framework has no equivalent to the 25% non-diversified basket, a manager of a dual-strategy fund cannot use that flexibility. The entire portfolio must effectively comply with the UCITS 5/10/40 rule.

Imagine a manager wants to invest heavily in a company they believe is undervalued.

• A US-only '40 Act fund manager could invest up to 25% of the fund's assets in that single company.
• A UCITS manager is capped at 10%.

For a mirror fund to be compliant in both regions, the manager is bound by the 10% UCITS limit. The high-concentration strategy available under the '40 Act is off the table.

These rules force portfolio managers to make strategic decisions not just based on investment merit, but also on the regulatory container in which the strategy is sold. The constraints fundamentally shape the risk and return profile of the fund available to investors in different parts of the world.

Understanding these diversification rules is key to analyzing how global funds are constructed and managed. The seemingly small differences in regulatory text create significantly different investment landscapes for managers and investors alike.