PD rating generally refers to the rating of private equity investment companies or private equity funds, also known as the rating of private equity investment institutions. Mainly by professional rating agencies to evaluate the performance, risk and management level of private equity investment companies or private equity funds comprehensively, so that investors can understand their investment value and risk level. PD rating usually adopts letter rating system, including AAA, AA, A, BBB, BB, B, CCC, etc. Among them, AAA is the highest rating, indicating that the private equity investment company or private equity fund has performed well in performance, risk and management. CCC is the lowest rating, indicating that there are great problems and risks in the performance, risk and management of the private equity investment company or private equity fund.