What is the Cable Select?

You may see the MA, SL and CS pins on a installing jumper. As you know the MA is master and the SL is slave.

Then, what is the CS?
The CS stands for Cable Select.

If CS jumper is selected, the HDD can be Master or Slave. But if you need this function, you need a special cable, which support CS jumper selection. The cable has two connectors and each one’s function is already decided as Master or Slave. By directly installing the CS jumpered HDDs to the cable, you don’t need to adjust jumper pins on the HDD.

Why use the CS and for whom?
The CS is useful for mass production computer(PC) maker. Because, if they put more than two HDDs into their product, they must set a jumper(shunt) pin for a slave. It would be a big loss to produce themselves. But it is not useful for end users, because an end user must set a jumper pin anyway, for using a cable with the CS.

source: hddoctor