出典:Wiktionary
Suao
In the context of Hanyu Pinyin, Suao can be considered a misspelling of Su'ao. In theory, a syllable-dividing mark (隔音符號/隔音符号 (géyīn fúhào)) should be added before a non-initial syllable beginning with a, o, or e. Hence, Suao is not allowed since a word made up of su and ao would be spelled as Su'ao (cf. Su'ao) and a word made up of su, a and o would be spelled as Su'a'o. In practice, syllable-dividing marks are often added or omitted at will.
Su'ao