I liked the question so I did a bit of searching. It appears that "Combustable" means that it can burn and will ignite but its flashpoint is above 100° F (37.8° C) and "Flammable" means the item is easily capable of ignition and has a flashpoint less than the above. However, that it where you should ask for guidance - short of trying to set fire to passengers bags to see how readily they ignite...
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