The Work Breakdown Structure
The WBS is the number-one most important planning document. But why? Because the WBS is utilized to organize 100% of the scope of work as represented in the currently approved project scope statement. If the scope statement changes, then the WBS will be updated. Understanding how they both work together can help you understand why they are the most important scope and planning documents.
Imagine if somebody came to you and said, "I want you to set up helpdesks in all 87 of our corporate locations in 15 countries, train everyone, and do it in a timely fashion." That is a massive scope of work! It would be impossible for you even to comprehend that much information, let alone create a schedule and budget to meet the requirements right away.
It is a good assumption that much of the scope of work can be duplicated in each location, or at least the processes that you use can be similar across locations. But you will have multiple stakeholders...