In a multicore cable, what is the nylon "string"?

I am using 4 core shielded cable. However, there is a strand of what I assume is nylon running parallel to the conductors. A bit like this

(source)

What is it for?

Answer

That string can be used to strip the jacket off of the cable.

You nick the cable at the end, put the string in the cut, then you can pull the string back and it will cut through the insulation on the way back.

For tough insulation (or soft fingers!) you might want to to tie a loop in the end first.