## 3 Research Methodology
### Category Selection  
The review is structured and constructed based on the goal of addressing the existing works in a systematically manner, as well as determining the research gaps and trends of works within the scope of the study. Furthermore, in order to integrate the study with existing and future works, existing classifications are noted, compiled, and improved where needed, complimenting previous studies, especially previous review papers that highlight certain gaps within the field. The collected materials are then classified according to the selected classifications that are guided by definitions in the literature. In this study, the definition of a disaster is as in [8], which is defined as an extraordinary event that could be caused naturally or due to man-made activity. The disaster that could occur with or without warning may also lead to another disaster bringing destruction, despair, loss, and suffering at a scale that could not be addressed by common emergency services. Based on this definition and guided by the motivation of the study, the materials collected are classified in details, based on the following attributes:  
1. Application: specific humanitarian operations addressed through VRP. 2. Disaster: types of disasters and phases of DM. 3. Modelling: characteristics of model. 4. Solution approach: groups of methods to solve the problem. From these attributes, classifications are then performed in two stages sequentially:  
1. Classify papers addressing VRPs based on the applications of three specific humanitarian operations (attribute 1). 2. Classify the resulting papers under the three umbrellas further in terms of disasters (attribute 2), model characteristics (attribute 3), and solution approaches (attribute 4). The three specific humanitarian operations that are observed in the literature are:  
1. supply and delivery operation,
2. evacuation operation, and
3. rescue operation. Through the first stage of classification, this study complements the works of [7, 8], which reviewed the applications of VRP in disaster, focusing solely on the delivery of the relief supply. The same could be said for [10], which focused on broader OR operations to address the casualty management. These three applications of VRP in humanitarian operations are further detailed by specifying which disaster phase that each paper addressed in the second stage of classification. These phases are classified into four categories, as proposed by [23] and applied worldwide: (1) Response, (2) Recovery, (3) Preparedness, and (4) Mitigation, under Disaster Phase (DP). Furthermore, disaster type is also included for classification labelled as \"Typ\" consists of: (i) rapid onset disaster and (ii) slow onset disaster. Through these additional classifications, specific insights in terms of trends could be obtained in relation to the nature of disasters addressed as well as the DM perspectives of the selected humanitarian operations. In terms of attribute 3, the papers are further classified based on the modelling aspect of complex or rich (R) VRP, deterministic (D) VRP, stochastic (S) VRP, dynamic (Dy) VRP, as well as multi-objective (MO) VRP. Ref. [17] is referred to for the definition of the stochastic and dynamics in the classification. Meanwhile, the definition of RVRP is based on [12]. However, the extended and complex version of typical variants of VRPs, such as CVRP, VRPTW, and VRPPD, are also considered to be rich. The classification of RVRP resonateswell with the suggestions from [7; 8; 9] and ref. [15], which emphasised the importance of RVRP. The DVRP is also included in the classification due to the large number of papers collected addressing such routing problems. Moreover, the classification for DVRP is added in order to investigate the possible trend of research within the context of this study. As is often included in other review papers, the multi-objective attribute of modelling is also of importance and applied in many review papers. As such, the classification for this attribute is also performed.