A Notice of Understanding and Intent, often called a NoUI, is the opening move in any serious correspondence. It is not a letter of complaint and it is not a request. It is you setting out, in writing, who you are, the capacity in which you are willing to engage, and the terms on which any further dealing will proceed.
The point of a NoUI is to put the other side on notice. Once served, they cannot later claim ignorance of your position. If they continue to write to the legal person and treat you as that fiction, they are doing so with full knowledge that you have separated yourself from it in writing. That is a different posture, in lawful terms, from someone who has never raised the question.
A NoUI typically contains four sections. First, your declaration: who you are as a living man or woman, the name you answer to in the lawful sense, and the fact that you are not the legal person whose name appears in capitals on their records. Second, your understanding: a plain statement of how you read the situation between you and them. Third, your intent: the terms on which you are willing to continue to communicate, and what you require from them in return. Fourth, the conditions for tacit agreement: a statement that if they do not rebut your notice point by point within a stated period, usually twenty-one or twenty-eight days, they are taken to have agreed to it.
Tone
A NoUI is not aggressive. It is calm, written in the first person, signed by hand in blue ink, with your autograph rather than a corporate-style signature. There is no shouting, no capital letters across paragraphs, no insults. The strength of the document is in its clarity, not in its volume.
Hold the line
The other side will most likely ignore the document on first sending. That is expected. Their staff are not trained to engage with notices framed in this way, and their software does not have a field for it. Your job is not to make them respond. Your job is to build the paper trail. Send by recorded delivery, keep the receipt, keep a stamped copy for yourself, and note the date in your own log.
If they continue to write to the person and demand action, your next letter refers back to the NoUI by date and reference number. You are no longer in a fresh exchange. You are in a sequence, and the sequence is on your terms.