Complex sentence: There is a fourth and highly exclusive level known as Invitation Only or'IO'--although IO is not listed on the Emirates' site the airline acknowledges its existence but does not publicly talk about this level or the specific requirements for entry.
Split into simpler sentences: There is a fourth and highly exclusive level known as Invitation Only or'IO'. While IO is not listed on the Emirates' site the airline acknowledges its existence but does not publicly talk about this level or the specific requirements for entry.