Normally, only Unconscious is Helpless, though I think that perhaps someone who is restrained AND immobilized might also qualify. I can't point to any rules on that, but if you can't coup de grace a completely bound, tied up prisoner (because they aren't HELPLESS), then I don't think coup de grace works (and there would be no reason not to simply say you can coup de grace Unconscious people).
Because coup de grace isn't limited to just Unconscious people, I think it, by design, is supposed to require DM judgement.
Is the person restrained and immobilized because a goblin is grabbing him and he's affected by Carrion Crawler poison which he could save against in his next turn?
Is he restrained and immobilized because he has been tied with rope to a stone pillar, with his hands tied together first, and then his legs bound together, and then securely tied to the pillar?
The first one seems like a poor excuse to use Coup De Grace. Even if you can't move from your square, you might still stagger around a bit. Even if you are held by the goblin, you might lurch away from the killing blow. Furthermore, you are only going to be in this situation, potentially, for 6 seconds. On your turn, you might break free from the restraining goblin, or save from the poison, or both.
In the second case, you might presumably have tried, repeatedly, to escape, and failed. You might be, quite literally, unable to move at all. The coup de grace is much more plausible here.
By definition, both situations are Restrained and Immobilized, but I think you would want to handle them differently. I think that WOTC wants to give you that freedom to do so, and recognizes that they probably can't spell out, using conditions, exactly what Helpless is.