spool_lock_filespool queue lock file - default %P
When the lpd server starts printing, it will fork individual worker processes to service each queue. To prevent multiple processes from working on the same queue, a printer lock file specified by the queue_lock_file option (default %P - the %P is expanded to the print queue name) is used. In our example, the lock file would be: /var/lpd/pr/pr.
The process ID of the currently active printer is stored in the lock file. By reading the lock file and testing to see if the process is still active, programs such as lpq can determine queue activity.
Similarly, the worker process may need to create other processes to assist it. These in turn will create lock or temporary files in the spool directory as well.