Below the Transform controller is the Rotation controller. Rotation is a data type that can use most of the standard controllers such as TCB, Linear, and Noise.
Rotations in 3D are very complex. Even the standard controller types behave a differently when used for rotation. The most common way of calculating rotations in 3D animation uses four components to define rotation about an arbitrary axis. This is the quaternion method.
The benefits of using quaternion rotation include:
It produces a direct one-to-one relationship between the quaternion values and how objects interactively rotate in the scene.
It produces smoother rotation than other methods.
The drawbacks of using quaternion rotation include:
Rotation values in key information can be difficult to understand.
Quaternion rotation controllers do not display function curves in Track View.
You can use the following controllers only as rotation controllers.
Use Smooth Rotation when you want rotation to have a smooth and natural look. Smooth rotation uses nonadjustable curved interpolation, and has the following characteristics:
You can move keys in Track View to change timing.
You can directly rotate objects in the viewports to change rotation values.
You cannot display controller or key properties, or function curves.
Use the Euler XYZ Rotation controller when you do not want to use the quaternion method. Euler XYZ is a compound controller that combines separate, single-value float controllers to specify an angle of rotation about each of the X, Y, and Z axes.
Euler XYZ is not as smooth as quaternion rotation, but it is the only rotation type that you can use to edit rotation function curves.
Euler angles are well suited for animating mechanical rotations. They can also be a good choice for an object in an inverse kinematics chain because IK Rotational joints are defined as Euler angles. X,Y and Z function curves displayed by this controller in Track View represent rotations in "world" or "parent" space which is the rotation system used by most 3D animation packages.