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Most of the black holes are believed to be rotating, since they are formed from collapsing stars, and all stars known to be rotating.
Rotating black hole is measured by mass and spin (0 to mod [1 - max])).
Additional matter, falling into a black hole also contributes to its rotation.
The mass of a black hole is estimated by its influence on other far bodies. Spin is harder to measure than mass, as it can be measured only with the closest particles to the black hole.
Innermost Stable Circular Orbit (ISCO), closer than this no orbits are stable, and falling into a black hole. The faster it spins the smaller the ISCO becomes. The spin is supporting the particles against the gravity.
The greater the spin, the closer the ISCO shrinks, up to the event horizon. ISCO’s less than event horizon haven’t been seen yet.
So, ISCO and black hole spin are related. One of the closest to the maximum black hole spin detected is 0.98 (GRS 1915+105).
The radii of ISCO discs are measured in the same way as the stars are measured.
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