Great tool for tracing the pattern and helping cutting the fabric.
The blue pen is water erasable pen
The purple and the pink pen are air erasable pen
They work well on light colour back of the fabric, especially on knit fabric.
I found it is hard to use chalk on stretchy loose knit fabric, but these pens are just perfect for knit fabric.
There is another option for dark colour knit fabric. It is called china marker originally made in Korea. The texture is between crayon and pencil. I have used them for dark colour knit and they were great.