A startup bean is a session EJB that loads and runs before an application starts. The Branch Transformation Toolkit uses the startup beans to do the initialization for some of its components, such as the CHA, CHA Formatter service, and services. Startup beans can also do clean up when the application stops, though the toolkit does not use the startup beans to do clean up.
It is the WebSphere Business Integration Server Foundation that provides the runtime environment for the startup beans. You can find more information about the startup beans from the product documentation of WebSphere Business Integration Server Foundation.