Answer
A best efforts offering is one in which the investment banker acts only as a distribution agent.
Underwriting and the best effort offering are two separate things. According to this type of agreement, the investment banker just serves as an agent for the firm issuing the shares and is not held responsible for purchasing shares in the event of a nonsubscription.
The underwriters do not commit to acquiring all of the securities from the issuer in a best-efforts offering. When marketing the securities to investors, underwriters commit to acting only as an agent of the issuer while making every attempt to sell the securities. When an underwriter consents to use every reasonable effort to sell as many IPO shares as possible.