Given that the product is vegan, either plants or fungi. Beyond that, I have no idea how you'd know. In the US, the Food and Drug Administration defines it thus

Given that the product is vegan, either plants or fungi. Beyond that, I have no idea how you'd know. In the US, the Food and Drug Administration defines it thusly: The term natural flavor or natural flavoring means the essential oil, oleoresin, essence or extractive, protein hydrolysate, distillate, or any product of roasting, heating or enzymolysis, which contains the flavoring constituents derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat, seafood, poultry, eggs, dairy products, or fermentation products thereof, whose significant function in food is flavoring rather than nutritional. (https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/cfrsearch.cfm?fr=501.22 https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/cfr...)