The forehand grip is mainly used for forehand overhead shots. It’s an easy grip to learn, and also provides a useful reference point for learning the other grips.
The panhandle grip is mainly used for forehand shots in front of your body. That means it’s useful for many forehands in the midcourt or at the net, but not in the rearcourt.
The thumb grip is like the backhand version of a panhandle grip. It’s mainly used for backhands where the shuttle is in front of you. That means it’s useful for many backhands in the midcourt or at the net, but not in the rearcourt.
Often we need a less extreme version of a thumb grip. I call it a backhand grip, because that’s the most common name. You could also call it a partial or moderate thumb grip.
The neutral grip is mainly used in between shots, as it helps you change quickly to other grips. It can also be used for hitting certain shots.