IT systems architecture is not a prescriptive or fixed in stone document outlining every detail of the build of a system but the encapsulation of the overall objective in generalised terms. It is akin to a business plan in that the goals, objectives and outlines of the business are described but the day-to-day mechanics of implementing the plan is not.
Everyone takes a slightly different slant on the systems architecture with the client concentrating on the functional requirements and user experience, the engineer on implementation, the project management on the bill of materials, functional and non functional requirements and the security and risk team with information about what will be stored and how this will be stored/accesses/transported.
The service agents need to understand how the system is to be used by the end users, what services are provided and what disruption if any they can tolerate. Finally the project managers and finance officers need to understand the approximate times it will take to implement and deliver to plan and budget accordingly.