Whenever gods have a child with a human, it was the gods choice to claim that child or not. This was a sign that the demigod child was a son or daughter of a certain god. At Camp Half-Blood, being claimed is considered a great honor and they use this knowledge to sort you into the cabin of your godly parent. Before the events of The Last Olympian, not every god had a cabin so a claimed child of a minor god would end up in the Hermes' Cabin.