What is the function of the nthreads parameter in iml.iml action?

The nthreads parameter specifies the number of threads to use on each node. This parameter is used for parallel computations. When set to 0, all available threads are used. In some circumstances, the iml action might use fewer threads than the value that you specify in this parameter because the action's internal algorithms determine that a smaller number of threads is preferable. The range for this parameter is 0–128.