What does "just below the sternum" mean with regard to cardiac issues? does that mean upper stomach below rib cage? or under direct center of chest?

Just like it sounds: The sternum is the breastbone - a vertical bone in the center of your chest. You can feel the notch at the top, a few inches below your neck. The bottom of the sternum is a couple of inches below where your nipples are - that is called the xiphoid process. Here's a picture: http://diseasespictures.Com/wp-content/uploads/2012/09/sternum-pain-4.Jpg. Cardiac chest pain can occur there.

Maybe: You dont give us any clues so there are two possible interpretations. The sternum is the breast bone so, in the vertical direction, just under sternum is the top of the upper abdomen. But when people have chest pain in the middle of the sternum we sometimes call it substernal, meaning under or inside the sternum in the horizontal direction.