The event builder orderer stage core is a loadable Tcl packge. Non time-critical functions, such as connection management are handled by Tcl scripts while time critical operations are compiled C code with Tcl command interfaces.
The EVB::handleFragments indicates that a file descriptor (usually a socket) has event fragments available. The command will read an FRAGMENTS flattened fragments body from the file descriptor and submit those as event fragments to the event orderer core.
The format of the data expected on that file descriptor is described in eventorderer.