combinable provides thread-local storage for holding per-thread subcomputations that will later be reduced to a single result. It is PPL compatible.
enumerable_thread_specific provides thread-local storage that acts like an STL container with one element per thread. The container permits iterating over the elements using the usual STL iteration idioms.
This section also describes template class flatten2d, which assists a common idiom where an enumerable_thread_specific represents a container partitioner across threads.