I had a fair bit of those over the last few years - they go though if there are unsold seats (they are not related to any booking class bucket).
The curious bit is that I had some go through at T-24, others went though a couple of weeks before the flight and some at the airport check-in desk on the day of departure.
A couple of times they didn't go through because flights were oversold in each cabin - I was able to use them on following flights.
Best value I would say is W to J or J to F - on ultra long haul (SIN, SYD, etc.).