Imperium of Man (the Aquila symbol)

Q: "This is the official symbol for the Imperium of man but is there an official flag?" A: "Each Imperial sector, world, and nations on those worlds have their own flags. Thus there is no need for a modern flag for the Imperium as a government, everyone just sees the Aquila nearby and goes "Yep, Imperial."" (https://www.reddit.com/r/40kLore/comments/4qab85)