Every limb in CAT has an IK target built in. By default, in the case of legs, the IK target is displayed as a rectangular outline called the FootPlatform; on arms it is a cross-shaped Point helper. The latter is not present by default; it must be added with the Create IK Target command.
Because of their function, IK targets are not linked to the rig, though they are transformed by the Layer Transform gizmo.
The only difference between the FootPlatform and the IK target for the arm is that, while keyframing, the FootPlatform allows you to animate the pivot position manually with the Foot Pivot system.
CATMotion has controls to animate the position of the pivot throughout the motion cycle. Normally you would want it at the toe as the foot lifts and at the heel as it plants, but the ultimate decision lies with you.
By default, the local Y axis of the FootPlatform points forward and the local X axis points up.
The FootPlatform on the Base Human rig (toes added)