When you design or develop your system, there might be three or more different sources of information that together describe the behavior of your ARM target. Depending on the nature of your question, you might have to combine two or more of these to cover all aspects of a specific behavior.
For example, if investigating a memory corruption problem on a target, you might have to combine information from documentation that describes:
the ARM processor
the bus interconnect used in the device
the external cache controllers
the memory controllers
the memory devices.
Hopefully this article has helped in providing insight about where you are likely to find answers to questions that arise from your ARM-based design.