A string specifying an item prototype flag.
"draw-logistic-overlay" | "excluded-from-trash-unrequested" | "always-show" | "hide-from-bonus-gui" | "hide-from-fuel-tooltip" | "not-stackable" | "primary-place-result" | "mod-openable" | "only-in-cursor" | "spawnable" | "spoil-result" | "ignore-spoil-time-modifier"