I have a question to a ticket from LOT, the class is U. I don't know, does it belong to Economy Flex or Saver or Promo ...?
Can it be cancelled freely or for a fee?
What is the fee?
The only thing I can find out is that 'U' refers to the distance of 125 mil which can be credited to your Miles and More programme.
Cancellations depends on too many things: how many days before the flight, method of purchase,...you should ring the airline and ask.
All I could find was that 'U' refers to economy class tickets, no category within that was specified.
On LOT's US site, it states that tickets can be cancelled without charge so long as the booking was made no more than 7 days previously.
Your best bet would be to contact them directly.