I'm always intrigued by this subject, so on Sunday on my way back from Milan on a Club Europe redemption ticket (U) connecting to ABZ, I asked the question just for fun at the South Galleries Lounge.
"You only get access on a U ticket". I am silver so not a big deal this time, but I persisted saying it was stupid that only redemption Club tickets got access, and he looked up his rule book. "Yup, only U class ex Europe and ex any long haul.
Words fail me with all the different answers people get.