Did some random search on CX.com and notice that some flights have J upgrade inventory (in U class) available but no J class award seat on that same flight.
Can I conclude that they come from different inventory?

When you said award seat, I suppose you meant full redemption instead of upgrade. They are both U. I always think they are from the same inventory, but may have different priorities for waitlist. Anyway, as you probably know, CX.com is not always accurate.

I've seen instances when CX.com was showing a J->F upgrade (Z) on a particular flight, but no F reward ticket. Presumably they're "different" Z buckets.

I think I have seen somewhere that the priority is as follow, although all in U class:
1) Companion award
2) Upgrade redemption
3) Full ticket redemption

my recent (a month ago) experience with this different bucket issue was reported in post #3 here:

http://www.flyertalk.com/forum/showthread.php?t=698042

Is there a master thread somewhere listing the fare codes for all 3 classes (F, J, Y), and the various fare buckets (revenue by descending order, full redemption, upgrade award, companion award).

Based on my search, I see that economy is: YBHKMLV (soon YBHK are only upgradable fare)...

For business, revenue is JCDI. Redemption is U (full, upgrade or companion award)...

First: F, A
Business: J, C, D, I
Economy: Y, B, H, K, M, L, V, S, N, Q, O

Awards (regardless of how it's redeemed, i.e. companion, upgrade, full redemption have the same fare codes)
First: Z
Business: U
Economy: T

But like others have said, I have noticed that even though companion, award and full redemptions come from the same booking class, I have experienced circumstances in which upgrade Z was available but full redemption Z was not.

Thanks, kchika, this is great.

A related question: does there a listing/website showing the above information for all (or other major) airlines?

Also, I understand that seatcounter.com shows seat inventory by fare codes, but I believe it only has revenue fare codes and not redemption fare codes. Is there a similar tool showing inventory by redemption fare codes?

Ok so my suggestion is this: Call up and waitlist the flight as a Companion award (and if neccessary, book a companion seat as well in anyone else's name). No need to issue, just book.

As your given highest priority, once you have your U class seat confirmed, cancel the companion booked seat, and tell them to intend to issue it as either an upgrade redemption or a full ticket redemption. This is your right how you finally issue your ticket, once you have the U class seat you can issue it anyhow you want to.

To increase your chance of getting a confirmed seat, ask any diamond card member friend of yours to book their seat in J class and you the companion (with diamond priority of course). I recon you do this, and if there is any award seat available, you'll be the first to get it!

Companion does NOT get the same priority as the member. I have doubled checked this already.

how sure we are with this ? at least this is definitely not what the MPO agents said to me every time I booked a companion ticket. they always make sure I understand it very well that I'm sure I want a companion award (double/triple asked a few times), because they said these have different availabilities as normal full award and even if I get confirmed on a companion award, if in the end I want a full award instead, I need to check availability for full award at that point. but, that said, I haven't tried this though, changing from companion to full award in the end...

and, from my experience, it doesn't sound like my companion gets diamond priority, at least not automatically.

tedhl is coreect. I have once changed a full redemption to an upgrade and the CSO has to ask her supervisor to check whether or not that was possible. :-:

It should not be possible in general. It is equivalent to cancelling a full redemption and rebook an upgrade. I asked that once and could not be done.

However, having status will help in rebooking the upgrade though, if the member is the traveler.

I am not 100% sure for Companion, for in my experience before I just asked the agent to book me an U or Z class and they never asked me whether is it for redemption or upgrade. When the deadline comes I just call them can either issue the redemption ticket or give them my ticket number for the upgrade.

I agree on redemption/upgrade - they all seem to come from the same bucket as far as I'm concerned: I've switched backwards and forwards on a few occasions.