Originally Posted by AJLondon

Spot on.

YY is the airline code designator that IATA use when not wanting to restrict a ticket/segment to a particular carrier. As such these tickets should be valid on all carriers, however, this has to be confirmed by the carrier in question and the ticket endorsed over to them. These days it is much more common to see a TA issuing a YY tix rather than an individual airline.

I frequently use these YY tix when flying within Asia (between SIN,BKK,HKG mostly) as I'm never certain of which carrier I'll end up flying! Never had a problem with SQ/CX/TG/LX/SK atleast in using these YY tix.