Bowl gouges, as their name implies, are primarily used in the production of bowls, platters, dishes and other hollow forms produced on the faceplate - normally with the grain running across the blank (perpendicular to the lathe axis). There are many factors which may influence the buying decision, including the flute shape and the handle length. Some factors, such as the style and angle of grind can be altered by the user.
We would also ask you to note that the sizing of bowl gouges is somewhat anachronistic and is historically based on the flute width - but of an old-style gouge forged from square bar stock. Today, round bar stock is used in the manufacture of most bowl gouges so the old measurements cease to make much sense in terms of being able to measure the gouge to determine its size. To make matters more confusing - albeit in a well-argued attempt to rationalise the situation - American terminology has changed in recent years to the more logical approach of using the size of the bar stock to define the gouge size.