std::experimental::parallel::seq, std::experimental::parallel::par, std::experimental::parallel::par_vec

From cppreference.com
constexpr sequential_execution_policy seq{};
constexpr parallel_execution_policy par{};
constexpr parallel_vector_execution_policy par_vec{};

seq, par and par_vec are instances of the execution policy types sequential_execution_policy, parallel_execution_policy and parallel_vector_execution_policy respectively. They are used to specify the execution policy of parallel algorithms - i.e., the kinds of parallelism allowed.