Hmm, I don't think it's that simple, lallyr.

It means KLM are willing to sell at least 7 economy seats in any fare class. L, T and V will be available to book, but that's no guarantee that your travel patterns (and the country of booking and/or residence) will meet the fare rules required for these codes. The fact that they have at least 7 seats they want to sell doesn't mean they're available: they may be willing to overbook.

I think you're right, Z is the fare code generally used for WBC upgrades before the flight. In the lounge, I'm not sure - they may be able to use other codes. As for the GE/PE 'any code will do' hush-hush possibly-ex benefit, who know.