Key answer
Hard limits make a violating result unacceptable; preferences are goals to optimize after meeting hard limits. Consequences determine this, not forceful wording. A publication word limit may be mandatory, while length in an internal draft may be negotiable.
Full explanation
IETF RFC 2119 uses must/must not for absolute requirements and should for recommendations with possible justified exceptions. Borrow its distinction without requiring ordinary users to learn specification keywords. RFC 2119, Sections 1–3
FollowBench incrementally added content, context, style and other constraints across 13 models; most models' overall compliance fell as constraints increased. Sections 3–4 and Table 2
List the most important two or three conditions and allowed tradeoffs. For example:
Mandatory: dates, amounts and names match source material; preserve the customer quotation verbatim. Preferred: relaxed tone and approximately 500 words. Keep the quotation if a relaxed tone would alter it; cut repetitive background first if space is tight. Report conflicts between mandatory conditions for my decision.
Mandatory items set acceptance thresholds, preferences give direction, and “if” clauses specify tradeoffs. Do not turn every preference into a prohibition or assign every item a numeric weight. If AI may omit minor content, state its decision scope directly.
Check hard limits first, then compare preferences. Better prose cannot offset a wrong amount; missing a preference need not mean total failure. For budgets and file permissions, use actual system limits/permissions so written agreements match available actions.