Why it's flagged. A custom style in your transmittal has square brackets in its name, for example "[[Letter]]". The factory already reads it as "Letter", so nothing breaks.
How serious. Just noting.
How to fix it.
- In the transmittal's Custom styles, remove the brackets from the name.
- Keep the brackets in your manuscript, where they mark the paragraph:
[[Letter]].
When to leave it. It's tidy-up only. You can leave it.