Yeah good question, that's the trick isn't it! I often use that one in the context of talking about signals (events)... about aircraft flight deck signal hiera

Yeah good question, that's the trick isn't it! I often use that one in the context of talking about signals (events)... about aircraft flight deck signal hierarchies and types... like alert, caution, warning, critical, emergency.... Some signals are proportional and escalating, so if you miss them first time don't worry as they will sound again and again until you attend. Other signals will (can) only appear once and the next time (if there is one) is catastrophic. Also it fits into the topic of information overload and mental capacity for tracking things (Miller's 7 +/- 2). Lots of bad things happen because alerts are too aggressive and people turn them off! :)