3.4.4.3 Differences Between Old and New Tracing Mechanisms

The newer tracing mechanism is more general and so more powerful than the older one: it works for expressions as well as procedure applications, and it implements the useful trace-subtree behaviour as well as the more traditional trace-here.

The older mechanism will probably become obsolete eventually, but it's worth keeping it around for a while until we are sure that the new mechanism is correct and does what programmers need.