What we call a "filter" is actually just a program. RPM Remote Print Manager® ("RPM"), from the beginning, has had the option to run a program of your choosing, with the command line you specify, using a file containing your print job as an option.
This comes from an episode in the late eighties or early nineties when I was consulting on a project. We found we needed a manual but the only copy we could find was a PostScript document. And our only printer was not PostScript capable.
The team leader gave me permission to install some open-source software on their system, so I grabbed an open-source PostScript interpreter, updated the "/etc/printcap" file on their Unix system, and in short order printed the manual by generating PCL suited for that particular model. It was really just a typical Unix moment, just the thing you learn to do ... and then mention casually to your nerd friends.
In case you didn’t know, some Unix systems have a configuration file called "/etc/printcap" which contains a list of the system printers. One of the capabilities of the Unix print system is to run a program with your job as input, then collect the output and do something with it, hopefully sending it to a print device. The general term for programs used that way in the Unix world is "filter", and that’s how the RPM feature got its name.