Interesting. I just recently booked a flight through Air Canada that includes the first and last flights being with Qatar Airways. Much the same, one of my legs have the special request code of ST listed as well. Being that it was only seen on the QR website and no where at all can it be seen on the AC website, I took to twitter to ask the QR support team. They told me that no special requests were showing on their system for the two QR flights, and that since I booked through AC, I should speak to their team. I contract AC on twitter, and they pretty much gave me the same run around. There are no Special requests showing on their system, and since it is only showing it on the QR website, then I should contact QR about it. I have even asked both airlines what the code ST means. QR has completely ignored the question it seems, and because my travel date is more then 3 weeks away, AC just replied with a Copy/paste reply directed me to a link for more information. The link provided is not relevant at all as it has only to do with the Max 8 groundings, and none of my flights are Max 8 flights.