First of, congrats to the people at LIGO.

[In this article](http://www.bbc.com/news/science-environment-36540254), the BBC notes that the latest LIGO results show that a new black hole was formed with a spin of $0.2$ (dimensionless number).

What exactly is this number?

Is this simply the ratio of the angular momentum that the blackhole is observed to have as a ratio of the maximal angular momentum as limited by some physics (Kerr Metric?)?