Forgive my ignorance, but what is HLN/HLO/etc.... tail number/plane identification? (oh oh... I googled it... lol...)

What's the logic behind the registration number/identifier scheme? Is that set by Cathay or the Hong Kong Civil Aviation Department? And on that note, does anyone know where Cathay registers and domiciles their aircraft? Are they owned by companies in Hong Kong? Registered in Hong Kong? Or some mysterious Caribbean island with questionable banking regulations? hehehehe....