When a segment of the fruit is rotten at a side, the whole segment usually has to be discarded. The fruit hasn't been ripening on the tree. Durian fruits will still ripen after taken of the tree. But the fruit once of the tree, will no longer develop the typical strong durian taste. Even if you are a durian expert yourself, you should, in Indonesia, always let the tukang durian do the choosing for you.
Durians are different in different regions.
Sometimes one can go by the smell of the closed fruit, but often one can't. And the tukang durian always has an edge over the buyer because he knows from which plantation, or even from which tree, his durians come from. I have considerable experience with durians. If I have been at a durian place for some time, I develop a feeling on how to select a good fruit. But still, instead of taking over myself the responsibility that a particular fruit I chose appeals to my taste, I rather let the seller do the choosing.