Before the solving step, goals must be converted to an IK/FK state so they can be used by the solver to output a skeleton state.
The IK state uses both the goals' rotation and translation and the FK state uses only the goals' extracted rotation. FK and IK effectors that don't have corresponding goals (because no markers were assigned to corresponding bones) simply take values from default stance pose.
Reaches can be adjusted to reach IK goals or keep the FK ones. When reaching IK translation, rotations up the hierarchy might be changed so that the translation is reached. For this reason, IK effectors have the same rotation as FK effectors so that if the rotation is kept, you can also reach this rotation.
For example, if translation is set to be reached on the IK wrist effector, solving can change the extracted rotation on the elbow so that the wrist reaches the right position. If the elbow rotation (i.e. its angle) is important, reach can be set on the elbow rotation so that it is maintained during the solve (which might result in the shoulder getting pulled for example). This is why IK effectors have the same rotation as FK effectors.
You can adjust the DOF Rotation properties during the solving setup process while the character is active. Previously, you could only modify the DOF Translation values post characterization.
Previously, you would set Degrees of Freedom (DOF) on properties on the resulting solve to prevent body parts from moving however, the results were inaccurate and sometimes undesirable. Now, you can lock the DOF properties on the skeleton and thererfore have the HIK solver faithfully follow the joint limits in the final resulting solve. This is called locking DOF channels.