As far as I know, there are (at least) three types of non-standard kanji usage now in existence in Japan: ateji, jukujikun, and gikun. There are also nanori, the readings used for personal and geographical names
My question is: does reading 月 as ライト in 夜神月, for example, fall into one of these categories? Aren't nanori, for example, also more or less fixed, as of now, in the range of their readings? Or are nanori only limited by one's imagination and are just a convention that remains open for additions?