The tail rig has two modes – FK and IK – if you select a tail conrol bone, you can switch it to IK mode, then it will be controlled by the IK bones (that look like arrows in a circle).
It works like this:
The mesh is deformed by a chain of bones in a second armature (tail rig) – this one has a spline IK
In the main rig there is an FK control set of bones (tail FK on bone layer 17)
In the main rig there is an IK set of control bones (which control the hooks of the spline IK tail rig – on bone layer 16 of the main armature)
setting the tail FK/IK slider all the way to FK tells the tail rig to copy rotation of the FK set of bones in the main rig
setting the tail FK/IK slider all the way to IK tells the tail rig to use the spline IK