if a location for a file starts with ssh: it is assumed the next part of the file is the hostname, followed by a colon, and then the path.
files will be copied using scp, and it assumes that you have a key on the server ( for passwordless access). You can set the user, port etc for the server in your .ssh/config file
if an ACL starts with ftp: it as assumed that the line is in the format "ftp:UserID:Password:someserver.com:/path/to/acme-challenge"
if an ACL starts with ftp: or sftp: it as assumed that the line is in the format "ftp:UserID:Password:someserver.com:/path/to/acme-challenge".sftp requires sshpass.
Note: FTP can not be used for uploading private key or certificates as it's not a secure method of transfer.
ssh can also be used for the reload command if using on remote servers.