## 2 Literature Review
[6] examined the CDP under a Drop-and-pull mode with three types of requests: IFER, OFED, and OE, restricting the same truck to picking up the separated container. The IFER request was further divided into two types according to the status of the emptied containers. If the emptied container released by the IFER request needs maintenance, it must be moved back to the depot; otherwise, it can be moved to any location where an empty container is needed or can be stored. They proposed a branch-and-price-and-cut algorithm to optimally solve the problem. Subsequently, [PERSON] et al. [64] added empty container constraints to their previous study. [PERSON] and [PERSON] [45] simultaneously modeled the CDP involving requests of IFER using foldable/standard containers, OFED, and IE under foldable state at the terminal and designed a LNS algorithm. The container type in an OFED request needs to be decided apart from the general decision variables in the CDP. When considering the inbound full or outbound full containers, the aforementioned studies always take into account the repositioning or pre-delivery of empty containers. However, due to limited equipment or time, sometimes an inbound full container may not be unpacked on the same day it is delivered, while an empty container delivered to the consignee may not be packed on the same day. Therefore, the basic requests, such as IF, ER, OF, and ED, should also be taken into consideration. [PERSON] et al. [5] constructed a generalized model for the CDP by considering six types of requests: IF, ER, ED, OF, IFER and OFED, but they assumed predetermined origins and destinations of all requests which reduced the complexity and applicability of the model. Similarly, [PERSON] et al. [11] also modeled these six types of requests, with the destinations/origins of ER/ED requests being flexible, but their DAOV graph treats the terminal node as a bypass node and thus does not accommodate IE and OE requests. Additionally, [PERSON] et al. [12] investigated six types of requests, encompassing five of the common requests (IF, ED, OF, OFED, and IFER), along with a special request type--IFOF. An IFOF request means a customer receives an inbound full container and immediately releases an outbound one, which can be easily split into an IF and an OF request because of no dependency on the operational sequence. Given that the terminal vertex must be visited for full container-related requests, they treated it as a bypass node in order to minimize the number of arcs. From the literature, it can be seen that many studies have investigated CDP with various types of requests, considering practical constraints (time window, capacity, and the availability of empty containers), trucking operation modes (Stay-with and Drop-and-pull),empty container repositioning strategies (Depot-turn and Street-turn), operational scenarios (multiple depots and multiple terminals), and the employment of new technologies (semi-autonomous truck platoon, electric truck, and e-commerce platform). While the pioneering work of the CDP and its variants focused on the pickup and delivery problem [13; 14; 15; 16; 17; 18; 19; 20; 21; 22], they did not differentiate between the detailed types of requests. To improve clarity, Table 1 summarizes the problem characteristics identified in closely related studies on the CDP, with a focus on differentiating request types. A comparison of the existing literature reveals that these studies bear the most resemblance to our research. However, the models for the CDP considering IE and OE requests usually duplicate the terminal vertex according to the request number, while those not including IE and OE requests typically treat the terminal vertex as a bypass node. To the best of our knowledge, there is no study that simultaneously incorporates the eight common types of requests into a single mathematical model.