This image pretty sums it up. When you pick your heraldry, the lord getting the "red and white" lozengy will display instead in the world map the "black hawk head on yellow". Didn't notice if it's just for castles and cities or also the lord's army. This does not occur when you pick the second "black hawk head on yellow" on the banner selection list, since there are two and the second seems to be unused by lords.
