Sorry if something similar has been posted before but I couldn't find anything when searching.
There is a PE fare MAN-MCO for approx £900 rt. Booking this with the intention of using miles to upgrade to UC. I can see from the Ts and Cs that only PE fare codes W,S,H,K can be used. When I look at the fare conditions of this £900 fare it shows the fare code as XLUG4SMW. This suggests to me that this is in fact an X fare. As far as I am aware X is an economy fare? I am wondering if they are doing something naughty and the UG in the code stands for upgraded and they are just selling an X fare with instant upgrade to PE?

Can anyone help with this? Thanks.

Edit: Matrix shows it as a K fare (upgradeable) but still confused by the X in the fare code on VS website.