Meaning of index parameter in hook function signature

[mattrossman]

Most of the hooks accept some function (e.g. PlaneFn, BoxFn etc.)
Each of these has a format like (index: number) => SomeProps.

What is the meaning of index in that function signature? I don't see this explained or used in any examples.

[stockhuman]

Better late than never?

You'll find your answer on L183 of hooks.ts:

props = uuid.map((id, i) => {
  const props = prepare(temp, fn(i), argFn) // here, fn is the x-Fn you mentioned
  temp.updateMatrix()
  object.setMatrixAt(i, temp.matrix)
  object.instanceMatrix.needsUpdate = true
  return props
})

It's a UUID index, used to iterate over every body in the scene.