Open Financials, then Claims, then the draft claim. Under the claim total, press Add a note to this claim.
Write what is about THIS claim: why the site was shut, why a line moved, what the builder asked for in writing last week. It prints on the claim, so write it for the builder, not for your own file.
Anything that is the same on every claim is not a note. Those are your trading terms. They go in Company admin, Terms, in the Terms on your payment claims card. Type them once and every claim carries them.
You can add or change the note while the claim is a draft. When you record the claim as served, the note is frozen with every other figure on it. Put anything further on the next claim.
Clear the box to take the note off. An empty note and no note are the same.