Directory used to temporarily store the documents to print and other printing related files. This directory needs to be writable (guess why) and large enough to hold the files until they are printed.

Note: You will see files in this directory only, if debugging is enabled. Else all files are invisible and get removed in a self-removing manner automatically if no longer used.; This directory must be writable and should use a filesystem which supports invisible files (refer O_TMPFILE flag in the open() syscall or caps_helper_invisible_file_create()), it must be owned by lp and group lp.

Refer Setup CAPS Document Storage Directory as well.