TAP has four products in Y, which correspond to the following booking classes:
tap|discount E T O N
tap|basic G W S L K U
tap|classic M H Q V
tap|plus Y B
X and I are probably award classes indeed.
I can sadly inform that there is no correlation at all nowadays regarding economy products and booking classes - I just booked Classic TXL-LIS-TXL and got the booking classes T and K, something the TAP booking page refused to show until I had paid the tickets, then it was suddenly clearly stated