The contents of these packets were free-form and were not defined in the B Plus documentation itself. However the basic concept was that the user's terminal program would respond to CIS's Interrogation Sequence (sent when the user first logged in) by starting a transfer with the M type. This stream would be used to send commands to the CIS host, which would respond by opening another transport layer stream back to the terminal program. These streams were "sequenceless", and read out in the order they were received. Errors or Failure packets caused both channels to abort.