If the context (top-most) element is "em", the "in-em" shortref map is current (as per the second USEMAP declaration), which defines the replacement text for "*

If the context (top-most) element is "em", the "in-em" shortref map is current (as per the second USEMAP declaration), which defines the replacement text for "*" to be "</em>", ending the emphasized text span. Whereas within "p", it's "<em>", starting an emphasized text span, and making "em" the context element.