Key answer
Their main purpose is to make relationships between parts easier to recognize. No symbol is universally optimal.
Full explanation
Punctuation separates conditions within sentences; line breaks separate independent information; headings distinguish tasks, material and examples; lists show requirements to handle individually; quotation marks identify terms, quotations and passages to edit. They resolve different ambiguities and need not all appear in every prompt.
Anthropic recommends meaningful, consistently named XML tags for complex prompts mixing instructions, background and examples. This helps structure recognition; it does not require everyone to write XML or guarantee security isolation. Official guide
This natural-language version is already clear:
Task: rewrite the customer's words politely. Customer's words: “You must refund me today.” Rule: change tone only; add no refund promise. Output: only the rewritten sentence.
Separating the quotation avoids confusing it with our instructions. If material already contains many quotes or headings, use explicit “material begins/ends” markers and explain their purpose.
Formatting-sensitivity research also finds that equal meaning can still produce different model performance. Easier human inspection and a higher score for a particular model are not identical goals; compare reusable prompts on actual tasks. Paper