std::experimental::parallel:: sequential_execution_policy, std::experimental::parallel:: parallel_execution_policy, std::experimental::parallel:: parallel_vector_execution_policy

From cppreference.com
class sequential_execution_policy { /* unspecified */ } ;
(parallelism TS)
class parallel_execution_policy { /* unspecified */ } ;
(parallelism TS)
class parallel_vector_execution_policy { /* unspecified */ } ;
(parallelism TS)

sequential_execution_policy , parallel_execution_policy and parallel_vector_execution_policy are tag types used to indicate the kinds of parallelism allowed in the execution of a parallel algorithm.