You should use IK Reach on your character when:

• You need to hold the positions of your character’s body parts during its animation’s interpolation
• You need to ensure that its body parts do not deviate from their positions at which their keys were set.

For example, keying your character’s foot placements when they contact the floor during a walk cycle. In this instance, setting keys on the FootEff effectors prevents undesirable foot sliding.

You should set FK Reach on your character when:

• You need to release its body parts from their held positions
• You need to key its positions, but the positions do not have to be exact during its animation’s interpolation.

For example, keying your character’s FootEff when its legs are off the floor swinging through the air during a walk cycle. In the instance, you need to key the movements of your character’s legs, but your character’s feet do not have stick to any one position.