[1] The first digit denotes whether the response is good, bad or incomplete: The requested action is being initiated; expect another reply before proceeding with a new command.
This type of reply can be used to indicate that the command was accepted and the user-process may now pay attention to the data connections, for implementations where simultaneous monitoring is difficult.
The command has been accepted, but the requested action is being held in abeyance, pending receipt of further information.
It is difficult to assign a meaning to "transient", particularly when two distinct sites (Server- and User-processes) have to agree on the interpretation.
RFC 2228 introduced the concept of protected replies to increase security over FTP communications.