I haven't used either command, but based on what I see in the manual, rcd tells rclone to start listening for remote commands whereas rc is used to issue remote commands.

Try it out by going to a folder with some files and typing: rclone rcd .

That should open a tab in your web browser with a list of your files.

There are situations where being able to send commands to rclone remotely would be helpful, but I'm not sure that you need to do that in this case.