For RML, this is the big picture for a large project that could be using agile
Business Objectives Model - Shows how the measurable goals of the projects relate to features
Key Performance Indicator Model- Shows how process flows relate to business throughput
Ecosystem Map - Shows how all the systems are interconnected
Business Data Diagram - Shows from a business point of view how all the data is related
I would say well-organized linked user stories are your best shot. You can also use a Functional Decomposition diagram.
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