High ratio flour is used almost exclusively by professional cake bakers, though it’s also found in store bought cake mixes. What makes this type of flour special is that it’s ground very finely and treated with either heat or chlorine (or both). What does that do? It allows the flour to disperse very, very evenly through the batter. That’s important because small, evenly dispersed flour granules gelate (“come apart”) more readily during baking, then tangle up with the egg proteins in the batter to create a very fine, tight mesh. This mesh is extremely strong and able to hold the cake up despite all the sugar. What you get is a firm cake layer with a tight crumb that rises high, yet still retains a moist mouthfeel, since high ratio batters are also moisture-heavy.