The spooled file number (SPLNBR) parameter is used when more than one spooled file is created by a job and the files all have the same name. The files are numbered, starting with 1, in the order that they are opened by the job.
The job log is always the last file for a job.
A file number is generated for each file when it is opened within a job (when output records are produced) and it is used by the system as long as the job and/or the files are on the system. If the files are not uniquely named because they were opened more than once, this file number is used to specify which file (or group of records, if the complete file has not yet been produced) is acted upon by a CL command.