As mentioned before a sub element can be a "transition element" or an "overlaid element". OTM displays the type of sub element in its overview track by a "T" or "O" indicator.
An overlaid element is a sub element that is completely overlapped by an element. The main or transition element that overlaps the overlaid element is called the "reference element" of the overlaid element, and the overlaid element is said to "refer to" this main or transition element.
A sub element that is not an overlay element is a transition element.