Barriers are programmed to accept various destinations.
At Liverpool Central, the barriers give 3 modes: 'No Geography' , 'Accept All', and 'Full Checking'. There is also a 'special rules' option that can allow various destinations that don't actually exist, (e.g. Southport to MRAIL AREA C).
No Geography checks the date and ticket type only, so will reject tickets that are out of date, or can't be used during the 'peak'. The ticket is NOT retained.
Accept all allows any ticket pass through without checking. It is NOT retained.
Full Checking checks the destination, date, whether it has been used before, ticket type. The ticket is retained.
Child Locks have three options too. 'Monitor' , 'Alarm', 'Reject'. The monitor option does nothing except makes the barrier display "CHILD" then opens anyway, alarm just makes them beep if a child ticket goes through, but opens the barriers, reject makes the passenger show the ticket to the gateline/RPI staff and sounds an alarm.
On the barrier, various codes appear to say what the problem with the ticket is:
Garbled, Unreadable, Invalid CHK, Out of Date, Not yet in date, wrong destination etc....
Next time your ticket gets rejected, have a look at the little screen underneath the green arrow, and I'll look the number code up for you.