It's the ignition temperature for paper. Fire burns at different temperatures depending on the fuel source, even paper will each a much hotter temperature after ignition.
It's an approximate and somewhat mythical value, popularised by Ray Bradbury's novel. Actual ignition point varies bu 100+ °F. Experience this past year with sourdough baking --- on parchment paper at 290°C (550°F) --- shows that though the paper browns, it does not combust. (Possibly affected by thermal mass / moisture of bread loaf.)
https://slate.com/technology/2012/06/ray-bradbury-death-does...
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