Quality requirements are high-priority information
arc42 suggests to document or specify the top 3-5 quality requirements in section 1.2 - right at the start of the documentation.
That’s helpful, as some of these requirements might influence the most important, fundamental (or often expensive…) architecture decisions.
System often have many quality requirements
In real-world systems we often find >100 different quality scenarios, way too many to digest all-at-once. That’s why arc42 proposes to document/specify this multitude in arc42 section 10…
See the following diagram for an overview where to specify/document quality requirements: