What items can domestic passengers carry on board?

Which items can domestic passengers carry on the plane? Passengers with first-class tickets are allowed to carry two items per person; passengers with business class or economy class tickets are allowed to carry only one item per person. The volume of each item must not exceed 20 * 40 * 55 cm, and the total weight of the above two items must not exceed 5 kg. Items exceeding the specified number, weight, or size must be checked as baggage according to regulations.