File beginning with “d”
The files whose names begin with a “d” are temporary files for active print jobs. These files contain the actual data of the file to be printed in whatever format as PostScript, text, PDF, etc. By default, these files should be removed approximately 30 seconds after a print job is completed, canceled, or aborted.
Files beginning with “c”
The files whose names begin with a “c” are “job history” files. They are retained until explicitly removed by an administrator with the CUPS cancel command.
The numeric portion in the names of both types of files is the print job number associated with the file. So, for example, the file /var/spool/cups/d19170 is the print data file associated with print job 19170, and the file /var/spool/cups/c19170 is the job history file for that same print job.
root# ls -lah /var/spool/cups/
--cut
-rw------- 1 root lp 821 Apr 13 08:52 c19169
-rw------- 1 root lp 819 Apr 13 08:53 c19170
-rw------- 1 root lp 819 Apr 13 08:54 c19171
-rw-r----- 1 root lp 276M Apr 13 08:07 d19170-001
drwxrwx--T 2 root lp 4096 Jan 16 10:16 tmp
root#