The spelling of the word "NIO" can be explained using IPA phonetic transcription. In IPA, "N" is pronounced as /n/, "I" is pronounced as /i/ or /ɪ/, and "O" is pronounced as /o/ or /ɔ/. Therefore, "NIO" can be pronounced as /ni.o/ or /nɪ.o/, depending on the emphasis placed on the vowels. It is important to note that the spelling of "NIO" is consistent with common English pronunciation rules, making it an easy word to use and spell correctly.