std::execution::seq, std::execution::par, std::execution::par_unseq
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header  <execution> | ||
| inline constexpr std::execution::sequenced_policy seq { /* unspecified */ }; | (since C++17) | |
| inline constexpr std::execution::parallel_policy par { /* unspecified */ }; | (since C++17) | |
| inline constexpr std::execution::parallel_unsequenced_policy par_unseq { /* unspecified */ }; | (since C++17) | |
std::execution::seq, std::execution::par and std::execution::par_unseq are instances of the execution policy types std::execution::sequenced_policy, std::execution::parallel_policy and std::execution::parallel_unsequenced_policy respectively. They are used to specify the execution policy of parallel algorithms - i.e., the kinds of parallelism allowed.
Additional execution policies may be provided by a standard library implementation.
[edit] See also
| (C++17)(C++17)(C++17) | execution policy types (class) | 


