FTP is used in some printing environments as a means to transfer a print job to a server, where it is then printed. The IETF (Internet Engineering Task Force) discusses issues of FTP printing as far back as RFC 448, "Print files in FTP". The article discusses files with unknown format (probably binary), plain text files and ASA formatted files.
HP offers FTP printing as an option in some of their JetDirect print servers. We found an article for the HP Jetdirect 510x print server which discusses FTP configuration. In the sample FTP session for this print server at the bottom of the article, we see that HP has advertised 3 ports. The ports are available as directories. In the example, they use the "cwd" command to change the working directory to one of the ports; then they upload the file. When finished, the print server will print the file on that port.
We also found a security article which suggests that using anonymous FTP (that is, no login or password required) to transfer files may result in printer abuse. We have heard the same concern from clients in the industry as well. If your print server offers FTP printing it may be worth asking the vendor about optional login support.