They are static values set in the defaults. If you open the Owen_IKAnimBlueprint in the Animation Content Example, you can see that section in the AnimGraph. If you take a look at the IK SkelControls in there, you can see the Joint Target for each is set to bone space and the bone chosen is the Thigh of each leg; so, the variables ‘Joint Target Left’ and ‘Joint Target Right’ that you point out are just offsets from the Thigh bone. As the character animates, the Joint Targets (pole vectors in Maya terminology) maintain that offset distance from the Thigh bone which keeps them in front of the character which, in turn, keeps the knees pointing forward.
Hope that makes sense. If not, or if you have other questions, just yell.