RAB Razor Fibre is a 1.18 mm string from RAB. It’s a thinner version of the RAB Sensor Fibre.
These are multifilament strings, so as they wear, the little filaments or fibers will break down and the string sweet spot will look like it’s slowly fraying. This is normal.
RAB Razor Fibre – Black
RAB Razor Fibre – Green
Thanks to Keir for the tip! Product photos are from stringersworld.com.