## 2 Research Status and Significance at Home and Abroad
### Model Building
To establish the joint optimization model, set the following parameters and decision variables:  
* Model parameters: \(V_{0}\): City Distribution Center; \(P\): Collection of candidate points of transit center; \(Q\): Set of end demand points; \(M\): Collection of package types; \(f_{5}\): Fixed costs of standard transit centers, such as site costs, sorting, loading and unloading, and handling supporting facilities; \(f_{L}\): Fixed costs of bulky transit centers, such as site costs, sorting, loading and unloading, and handling supporting facility costs; \(f_{MIX}\): Fixed costs of hybrid transit centers, such as site costs, sorting, loading and unloading, and handling supporting facilities; \(N_{p}\): The maximum capacity of the candidate point \(p\) of the transit center, expressed by the number of packages, related to the size of the package, and changes accordingly with different thresholds, \(p\in P\); \(d_{op}\): Distance from the city distribution center to the alternative transit center \(p\), \(p\in P\); \(c_{v1}\), \(c_{v2}\), \(c_{v3}\): Indicates the unit transportation cost of models \(v_{1}\), \(v_{2}\) and \(v_{3}\) respectively; \(cap_{v1}\), \(cap_{v2}\), \(cap_{v3}\): The maximum loading capacity of models \(v_{1}\), \(v_{2}\), and \(v_{3}\) are, respectively, expressed by the number of packages; for the same model, the number of packages under different large parcel threshold selections is different; \(w_{m}\): The weight of the \(m\) type parcel is converted into a joint measurement of weight and volume for light dumping with a large volume; \(C_{s}\): The unit handling cost of standard parcels, including handling and unloading costs and sorting costs in transit centers, loading and unloading costs at end demand points, etc., mainly depends on the average weight of standard parcels and is affected by parcel threshold settings; \(C_{l}\): The unit processing cost of bulky parcels, including handling, loading and unloading costs and sorting costs in transit centers, loading and unloading costs at end demand points, etc., mainly depends on the average weight of bulky parcels and is affected by parcel threshold settings; \(D^{Pick}_{i,m}\), \(D^{Del}_{i,m}\): indicates the number of packages collected and delivered by \(m\) type parcels at the end demand point \(i\), respectively, \(i\in Q\), \(m\in M\); \(D^{Pick}_{j,m}\), \(D^{Del}_{j,m}\): indicates the number of packages collected and delivered by \(m\) type parcels at the end demand point \(j\), respectively, \(j\in Q\), \(m\in M\); \(D^{Pick}_{Sta}\), \(D^{Del}_{Sta}\), \(D^{Pick}_{Ove}\), \(D^{Del}_{Ove}\): respectively, the total quantity of standard parcel collection and distribution at the end demand point, the total quantity of oversized parcel collection and delivery, and the threshold selection of different oversized parcels is also different; \(l^{v2}_{ij}\): the standard parcel volume loaded by vehicle \(v2\) from node \(i\) to node \(j\), including collection and delivery volume, \(i,j\in V_{1}\), \(i\
eq j\), \((i\cup j)\cap Q\
eq\varnothing\); \(l^{v3}_{ij}\): the volume of bulky parcels loaded by vehicle \(v3\) from node \(i\) to node \(j\), including collection and delivery volume, \(i,j\in V_{1}\), \(i\
eq j\), \((i\cup j)\cap Q\
eq\varnothing\);\(L_{p}^{s,Pick}\): indicates the total collection volume of standard parcels processed by the transit center \(p\), \(p\in P\); \(L_{p}^{s,Del}\): indicates the total delivery volume of standard parcels processed by the transit center \(p\), \(p\in P\); \(L_{p}^{l,Pick}\): indicates the total collection volume of bulky parcels processed by the transit center \(p\), \(p\in P\); \(L_{p}^{l,Del}\): indicates the total delivery volume of bulky parcels processed by the transit center \(p\), \(p\in P\); \(t_{i}^{v2}\): indicates the total time used when vehicle \(v_{2}\) departs from the transit center and arrives at node \(i\); \(t_{i}^{v3}\): indicates the total time used when vehicle \(v_{3}\) departs from the transit center and arrives at node \(i\); \(t_{ij}^{v2}\): the service dwell time of vehicle \(v_{2}\) at node \(i\) and the travel time from node \(i\) to node \(j\); \(t_{ij}^{v3}\): the service dwell time of vehicle \(v_{3}\) at node \(i\) and the travel time from node \(i\) to node \(j\); \(T_{v2}\), \(T_{v3}\): maximum working time of vehicle \(v_{2}\), \(v_{3}\) in a single work cycle.