There are hundreds of definitions about architecture available. Fortunately they have a lot of things in common. Personally I like the following definition:
“An architecture is the set of fundamental decisions that cannot be reversed without greater effort and impact.”
That definition focusses on the role of the system architect. It does not define the set of artifacts that are part of an architecture. It could be anything. This definition – as any other – is vague at the borderline. It does not clearly define what is part of the architecture and what is not. I’m pretty sure that you won’t find a useful definition that could clearly answer that question. Otherwise let me know
What does that implies for the practice? Simply spoken: Keep care of the interfaces! The borderline between System Architecting and Requirements Engineering, Systems Design, and any other specific engineering disciplines is blurry. Focus on the interfaces at these borders. And with interfaces I do not mean technical interfaces. It is about communication and processes.
For example the SYSMOD-Zigzag-Pattern explains the close collaboration between the Requirements Engineer and the System Architect. It helps to clarify the separation of both roles and defines the services provided and requested by each role. The requirements engineering must communicate the requirements to the systems architect. “Communicate” means really communicate and not just sending dozens of documents by email. The systems architect creates a system architecture that satisfies the requirements. She must communicate the architecture to the requirements engineer to assure that she has correctly understood the requirements. If you have another layer accordung to the SYSMOD-Zigzag layers, the requirements engineering derives requirements based on the architecture and the communication flow starts again.
Conceptually the relationships described above exist in every project. Whatever you explicitly apply the zigzag pattern with SYSMOD or any other methodology. It clearly shows that the requirements engineer and the system architect must collaborate and communicate as close and personal as possible.