Load modeling plays a very important role in power system modeling, and the load model is an indispensable component in power system simulation. To get accurate load models and formulate a unified document, this standard is developed to provide comprehensive policies and procedures of load modeling and simulations. A review and comparison of two existing most widely used methodologies for load modeling is presented in this document, that is, the measurement-based and component-based approach. A critical and updated overview of opportunities and challenges of load modelling with emerging networks and components is also provided. The guidelines for power system simulation with a variety of load models are proposed. A case study adhering to the proposed guidelines clearly indicates the need for a hybrid approach in the future that will combine strengths of measurement-based and component-based approaches with data acquisition capabilities offered by modern measurement equipment.