“To understand the big picture start from the outside”. Commence by understanding the target market and therefore the target audience i.e. end users of the system. Once you understand the “who”, you are ready to understand “what they would like to do” i.e. the “what” – the use-cases/requirements and then identify the technical features. Continuing on the same train of thought, it is necessary to understand the relationships of the various “whats” i.e. interactions/dependencies and the usage patterns (“when” & “how much”). Now you are ready to dig into the details of “how it is built”, ” where it is/will-be deployed” and the “what attributes” it should satisfy.
Understanding is best done “outside-to-inside” i.e start from “who” and then “how it should-behave/is-implemented”. The two core concepts of Landscaping and Viewpoints are very useful here. Landscaping is a technique to rapidly understand the system by examining the various elements and the connections between them. Viewpoints is a technique that suggests you look the system from the various end user points of view to identify the specific needs and the expectations that each end user may have.
Use a layered understanding approach as outlined in the picture below: