When CMBS loans are pooled together and securitized into commercial mortgage-backed securities, they are split into multiple tranches based on risk and return. CMBS tranches can generally be split into two major categories, investment-grade CMBS and sub-investment-grade CMBS.

The CMBS B-piece refers to the tranches of commercial mortgage-backed securities rated BB+/Ba1 through B-/B3, providing the highest risks and the highest returns for CMBS investors. If the underlying loans that back a CMBS go into default, the B-piece investors are the last to be paid back, if they get paid back at all.