Balance in our 3-D spacial reality refers to both sagittal and coronal planes. It means we can fall down to each of the four directions N, S, W, E or their multiple combinations. Since we are bipeds with an often used possibility to enlarge our base through spreading our legs wider apart (the bigger is the surface of base the easier is to maintain in balance) it is relatively easy to achieve the goal in the W-E axis (assuming we are facing North at address).
The true balance in golf, however, is surely being set in the coronal plane though. It seems to be quite an easy task when we stand erect motionless. When playing golf we need to be at inclined plane. It means that we need to learn how to counterbalance using our body parts (e.g. the head with the tush) so that the coronal plane always bisects the center of mass area. By the way, this famous losing of tush line issue is in fact nothing more nothing less but subconscious compensations for losing the overall balance, i.e. a subconscious reaction for displacing the line astray.
Note that the coronal plane line is set from the lead ankle joint up because this is where the vertical axis of downswing rotation (that corresponds to the impact phase) goes. Please also note that the pressure areas of both feet correspond with the coronal plane line allocation - as stressed in the diagonal stance articles - ankle joint area of the lead foot and balls area of the rear foot. Lastly, it says how important is replanting of the lead heel onto the ground for overall feel where the axis is.
Let's see how it looks further in the action: