What does it imply when something is referred to be a system model? What are some of the elements that must be addressed while developing core models?

The overall emphasis of the __________ phase is on gathering data on the existing system, determining the requirements of the new system, considering alternatives within identified constraints, and investigating the feasibility of alternative solutions. a. investigation b. analysis c. design d. construction

But what precisely is a "system model"? What elements should be considered while developing the first models?

It seems sense to want to define a system model, but what does it include, exactly? What considerations should be made while developing elementary models?