these are other RS232 protocol pins, specifically the hardware flow control pins. RS232 is a 50ish year old standard,
serial link from teletypes to modems and suchlike at 110 or 300 baud was the common application once. For some reason
a 25 pin connector was originally used, and there are many other pins defined I believe. These days noone uses all this
complexity. DTR or CTS can be used for other purposes such as reseting the Arduino remotely. They tend to be pulsed
automatically when opening the serial device on the computer.
Ok, so they have evolved from their original purpose but kept the names
No they haven't really evolved. They are getting used in a unique way.
and are some what a arbitrary serial pins the arduino is taking advantage off to perform functions like remote reset ?
No, not arbitrary. If you connect the adapter to a true RS-232 device, they will make use of the RTS/CTS handshaking. The arduino takes advantage of the fact that the pin goes low at a known condition. Again, it is getting used in an unique way.
CTS is an input to the FTDI chip, held low by the arduino to signal that the chip is Clear To Send.
Others have reported that the chip transmits when its not connected, so perhaps it is not used. I always connect it to Gnd on the arduino side.