Inputs
The Inputs section of the RBF Drivers panel is where you select which transform properties on the pose bone you want to use to drive things. You can select from location, rotation and scale channels, or any combination thereof.
Rotation Modes
All of Blender's driver rotation modes are also available for RBF drivers. This includes the swing/twist modes introduced recently. These can be especially useful in setups where you only care about the orientation of a bone, and not its Y-axis rotation. In this case you would select Swing and Y Twist and enable the W, X and Z rotation channels.
Note
If all rotation channels are enabled as inputs, the RBF driver will always use quaternions for the basis of it's calculations regardless of the rotation mode setting, as this provides the highest degree of reliability.