Sorry to have been travelling.
ASUM1Y is the fare basis that appears in the eTicket, it is not the booking reference. A is indeed the booking class.

When I booked on CX webiste, the fare rule said ticket valid one year.
However I now wish to change the return date and the CX agent tells me that it has to be flown before 30 September 2016. That was not mentioned in the description of the promotion on the website. It did say valid 1 year. They changed to text on the website to "return before 30 September 2016" after I complained. Unfortunately I did not print the old screen.