I have always looked at RTS (Request To Send) as a signal from a device to a modem indicating that the device has data and is ready to send that data, and CTS (Clear To Send) as a signal from a Device to a modem to indicate that it is now ok for the other end to start sending data. The usual sequence is RTS is asserted by the device wanting to send data and is followed by CTS being asserted by the receiving end when it is ready to receive the data.

Hal