It's not a metaphor, it's just jargon. It's how the total number of people on board an aircraft is clearly expressed without the confusion of "I've got 37 peopl

It's not a metaphor, it's just jargon. It's how the total number of people on board an aircraft is clearly expressed without the confusion of "I've got 37 people aboard" "Ok... is that 37 including you, or 37 plus the pilot?"