With the rising level of outsoucing of ground services by airlines (inc CX), I often find local check-in agents to be helpful but yet, unfamilar with the seat plan on the respective airlines' aircrafts. On some many occasions, I would ask to see the seat map myself so get a good/quiet seat. However, when I do see the seat map on screen I don't really understand what the symbols represent. These include "X", ".", "U", "C" etc..

Can somebody please advise whether these symbols are universal across all airlines and what they represent?

Thanks.