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In a large site, you could have several equivalent printers, which will be used by many people. The reason for this is, of course, to increase the printer output by enabling several jobs to be printed at once. A load balance print queue is one that feeds jobs to other queues and has a sv=q1,q2,... printcap entry that specifies the destination or server queues. These must be print queue entries and have spool directories on the server.
The service queues have a ss=mainqueue printcap entry This informs the lpd server that the queue operates under the control of the mainqueue print queue, and is fed jobs from it.
During normal operation, when the lpd server has a job to print in the mainqueue, it will check to see if there is an idle service queue. If there is, it will transfer the job to the service queue spooling directory and start the service queue printing activities.
Even though the queues are not meant for direct use, people can print directly to individual queues. This allows a specific load sharing printer to be used. If you wanted to hide the load sharing printers, i.e. - not allow direct spooling to them, then you would simply remove the non-server entries from the printcap.