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Ah, I get it. I thought that might have been a possibility, but I was also thinking that unusable meant that it was the amount of fuel required by regulations to fly to the destination, and thereafter for 30/45 minutes, and being marked as unusable.
But it all makes sense now, thanks!
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What you were thinking of is referred to as 'reserve'
If you read airplane test reports in magazines they will often list the range/endurance with reserve at different power settings. This is somewhat meaningless, as the reserves you are required to carry is different for day/night VFR and IFR.