It is an hybrid bank bond, that is convertible into shares, the option is not exercised by the lender (as in common convertible bonds), but it is mandatory to the occurrence of predetermined conditions. Co. Co. Bonds are converted into shares only if the financial conditions of the issuing bank. Following Basel III - Iinternational Regulatory Framework For Banks, these bonds are converted if the ratio of primary capital (consisting of the sum of the share capital plus profits and reserves less losses) and the total risk-weighted assets of the issuing bank falls below the threshold of 6%.

Bibliography
DE SPIEGELEER  J., SCHOUTENS  W. and VAN HULLE  C., The Handbook of Hybrid Securities: Convertible Bonds, CoCo Bonds and Bail-In, WILEYFINANCE, 2014.

https://www.bis.org/bcbs/basel3.htm

Editor: Giuliano DI TOMMASO