LSF translates signal numbers across different platforms because different host types may have different signal numbering. The real meaning of a specific signal is interpreted by the machine from which the bkill command is issued.
For example, if you send signal 18 from a SunOS 4.x host, it means SIGTSTP. If the job is running on HP-UX and SIGTSTP is defined as signal number 25, LSF sends signal 25 to the job.