What is the account field used for on an FTP Server, vs the username

Probably a stupid question, but when connecting to FTP servers, there is sometimes an option for account. What is the significance of this vs the username?

See this method from the Apache FTP Client for example:

/**
 * Login to the FTP server using the provided username, password,
 * and account.  If no account is required by the server, only
 * the username and password, the account information is not used.
 *
 * @param username The username to login under.
 * @param password The password to use.
 * @param account  The account to use.
 * @return True if successfully completed, false if not.
 * @throws FTPConnectionClosedException
 *      If the FTP server prematurely closes the connection as a result
 *      of the client being idle or some other reason causing the server
 *      to send FTP reply code 421.  This exception may be caught either
 *      as an IOException or independently as itself.
 * @throws IOException  If an I/O error occurs while either sending a
 *      command to the server or receiving a reply from the server.
 */
public boolean login(final String username, final String password, final String account)
throws IOException
{
    ....
}