ANSI Z535.5 Definitions:
• Danger: Indicate[s] a hazardous situation which, if not avoided, will result in death or serious injury. The signal word "DANGER" is to be limited to the most extreme situations. DANGER [signs] should not be used for property damage hazards unless personal injury risk appropriate to these levels is also involved.
• Warning: Indicate[s] a hazardous situation which, if not avoided, could result in death or serious injury. WARNING [signs] should not be used for property damage hazards unless personal injury risk appropriate to this level is also involved.
• Caution: Indicate[s] a hazardous situation which, if not avoided, could result in minor or moderate injury. CAUTION [signs] without a safety alert symbol may be used to alert against unsafe practices that can result in property damage only.
• Notice: [this header is] preferred to address practices not related to personal injury. The safety alert symbol shall not be used with this signal word. As an alternative to “NOTICE” the word “CAUTION” without the safety alert symbol may be used to indicate a message not related to personal injury.
The OSHA 1910.145 definitions for tags are as follows:
• Danger: "shall be used in major hazard situations where an immediate hazard presents a threat of death or serious injury to employees. Danger tags shall be used only in these situations."
• Warning: "may be used to represent a hazard level between "Caution" and "Danger," instead of the required "Caution" tag, provided that they have a signal word of "Warning," an appropriate major message, and otherwise meet the general tag criteria of paragraph (f)(4) of this section."
• Caution: "shall be used in minor hazard situations where a non-immediate or potential hazard or unsafe practice presents a lesser threat of employee injury."

Tips, Cautions and Warnings - Советы, знаки «Осторожно» и «Внимание»

Знаки «Внимание» указывают на необходимость особых мер предосторожности. Игнорирование этих знаков может привести к серьезным травмам или летальному исходу, значительному ухудшению работы системы или вероятности получения неправильных результатов анализа.

Указания со знаком «Осторожно» призывают к соблюдению соответствующих мер предосторожности или к другим действиям.
Игнорирование этих знаков может привести к небольшим травмам, снижению качества работы системы или ее поломкам, которые могут повлечь другие виды неблагоприятных последствий.
(Из неопубликованного :)