Extending vs. Modifying for design choice

Hi Guys,
I am trying to finish my design choice and now I am dealing with this part (Extending vs. Modifying)
I'd like to have your comments on this part (by the way I didn't used none of those

)
As I understand when we need to have a compelled new entity we will extends but if there is some functionality that is really bound to parent class we need to modify? But I guess I need some academic explanation for it,
Thanks again

Sam

It took me a while but I found my post from a few days ago. Check here for my response to that question.
Mark