Why does the decision come first?#
Because it sets how much must be understood. A reversible configuration change and an irreversible data migration need different depths of understanding from the same person, and starting with the decision is what stops every brief being written at the same length regardless of what rests on it.
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Why is decision impact more important than change size?#
A thousand-line refactor that preserves every interface may need less understanding than a three-line change to an authorisation rule. Sizing the explanation by the diff is the most common way to produce briefs that are simultaneously too long and missing the point.
The three parts map onto three artefacts: the decision queue entry, the delta, and the comprehension check.