I think you misunderstood the question. Modems transmit data. Assuming you are using TCP/IP, standard file transfer protocols include FTP (non-encrypted), FTPS (older encrypted protocol), and SFTP (new encrypted protocol utilizing SSH).
The modem data sheet says it supports FTP(S) - does that mean it supports FTP, FTPS, and SFTP?
We only need SFTP - so that is the one I am most interested in.