A spool request is a document for which a print function has been selected. However, it has not yet been output to a printer or another device. The output data for the print document is stored partly formatted in a data store until an output request is created, that is, until it is sent to a particular output device.
The spool system uses a spool request to store the print data temporarily and to access it. The data is stored in a temporary format. You can also display the print document.
The system automatically assigns a 10-digit ID number to a spool request.
Also Refer
http://www.allinterview.com/showanswers/71855.html
SAP document itself can explain you very well
http://help.sap.com/saphelp_nw70ehp1/helpdata/en/24/ece847a9d511d1a9e40020af00b7e5/content.htm
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