How CUPS get status of a job ?

[huy at cse-tech.net]

I am realy interested how CUPS get information of a job sent to my HP printer ?

on my print server CUPS sometime reports that a job has been completed however it is actualy not printed on the paper (may be just in buffer of printer or spooled on local hard disk)

As I know CUPS use Print Job Language (PJL) to sent a request to a printer and get back information from it ? But how it works and where is configuration file that specify printer command ?