Steve Naïve writes
I could understand most of them, but what's "54 - Possible Dumbbell"?

When only Z1 stations had automatic barriers; someone might buy just a
Z1 Travelcard to enter/exit the station even though their journey took
them to, say, a Z6 station. At the Z6 station they would hope not to be
challenged, or just pay the odd penalty fare. A Z1 ticket plus the odd
penalty fare is a lot cheaper than a Z1-6 Travelcard.

If a ticket is used to enter a Z1 station without a corresponding exit
from a Z1 station, the that code is displayed. The gates can be set to
reject such tickets, thus meaning the holder has to approach the SA to
get in/out. If the SA notices the same person with the same error code
then they can alert the RPIs. RPIs could then follow the suspect to
establish their travel pattern, arrange a station block at the Z6
station (again to establish a pattern) and then prosecute for each noted
offence of fare evasion.

Nowadays with more stations being gated, it means that the person might
need separate Z1 & Z6 tickets in order to achieve the scam. That just
means that the gates at either end of the journey can highlight the
possible dumbbell.

Been discussed here many times before.

--
Dave