How are exchange traded funds usually structured within the EU?

Within the EU, most mainstream exchange traded funds are established under the UCITS regime. UCITS is a regulatory framework designed for retail investor protection and cross-border distribution, with rules covering eligible assets, diversification, liquidity, risk management, disclosure, and custody. Because ETFs are commonly marketed to a wide investor base and traded on exchanges, being structured as UCITS helps them meet recognised standards and enables passporting across EU jurisdictions. Investment trusts are typically closed-ended company structures and are more associated with certain markets rather than being the standard EU ETF structure. Alternative Investment Funds is a broader category used for non-UCITS vehicles such as many hedge funds, private equity funds, and certain real asset funds, which are not the typical structure for vanilla ETFs aimed at broad distribution. SICAV is a legal corporate form used in some domiciles, but it is not the best answer because the question asks how ETFs are usually structured in regulatory terms within the EU, which is most commonly UCITS.