I was reading up on the etymology of hapa, and it originally meant half Native Hawaiian. Today, it has extended to mean half Pacific Islander or Far Eastern. If it didn't only mean half, then there wouldn't be words like quapa as well. Also, there has to be a cutoff point. For example, I think it would be silly to refer a person who is 1/16 Vietnamese and 15/16 Euro as mixed-race or part Asian.