boost::heap::arity, what is it?

I am cleaning a legacy code. Inside I have a priority queue from 1986 ^_^. After interfacing it with a C++ interface more and less compliant with the std. I did some benchmark between all priority_queues on the "market" (std + boost).

Boost provides a priority_queue name boost::d_ary::heap. This queue necessitates a parameter named boost::heap::arity, the documentation of Boost does not provide a clear explanation, just a link to the implementation of the heap.

Presently I put boost::heap::arity<128> I am really satisfied but I do not know what does it mean. One of you, have a bit of explanation ?

All the best,

++t