1. Tier 1 funds: Private equity funds can directly invest in Tier 1 funds, such as stock funds, bond funds and commodity futures funds. Tier 1 funds refer to fund products that directly invest in the underlying assets.

2. Secondary funds: Private equity funds can also invest in other private equity funds to form the investment structure of secondary funds. These secondary funds may have more specialized investment strategies, such as in-depth research and investment in specific industries, regions or asset classes.

3. Multi-level funds: In the field of private equity funds, there are also investment models of multi-level funds. That is, private equity funds can invest in other private equity funds, and these invested private equity funds can invest in other private equity funds, forming a multi-level investment structure. This investment model is usually used to manage and combine large-scale projects or complex investment strategies.