If you cannot write it down, you do not know it yet.
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Writing is not a way to communicate thinking that has already happened elsewhere. Writing is, for most working professionals, where the thinking actually happens. The fuzzy idea in your head becomes specific only when forced through the bottleneck of a sentence.
This is why teams that write a lot make better decisions than teams that talk a lot. The talk feels like progress and produces fewer commitments. The writing produces fewer feelings and more decisions.
Write the memo. The memo is the work, not its summary.