[GSoC / Question] TREEDEF vs built-in

Hi,

I'm implementing some OpenACC directives but since I'm a new contributor
with a very simple but conceptual question that I prefer asking in the 
mailing list rather than believing some unclear explanation from AI.

I know that some directives (Talking about OpenMP / OpenACC) 
imply futher optimizations or variables treatment and that case
is defined as a tree node (TREEDEF). In the other side, built-in 
is required when it's a runtime call that can be resolved in the
front-end (?).

I'm not sure about that and I would appreciate some examples. Not
necessarily in the case of OpenACC / OpenMP, but some useful examples to
judge if some statement requires built-in or define a tree node for that
(if requires special treatment).

Thanks!

Best Regards,
Sebastian Galindo