Why it's flagged. Inspect saw emoji or a special-purpose font. These may not print the way they look on screen, because the book uses its own fonts.
How serious. Just noting.
How to fix it. In any editor:
- Check the marked paragraphs in a proof.
- If you don't need the emoji or symbol, replace it with plain text.
- Tell the studio in the transmittal's Special Characters box if it matters.
When to leave it. If you can see it in the proof and it looks right, leave it.