Thank you all, BUT I have a bit diff conclusion. Per the table in the AA-link provided above by PHL
https://www.aa.com/i18n/customer-ser...vel-credit.jsp
For "Cancelled tickets" a Trip Credit is issued. Flight Credit issued ONLY for "Unused tickets".
I hope the table is correct!
Trip Credit can be used by holder for Anyone, but from my Personal experience for somebody else need to call - cannot be done online.
Probably not for BA flights. But we'll try...
Also from my personal experience:
The table in AA-link says for Trip Credit: "Valid for: Non-award trips starting in the US and select countries* ", the * is not elaborated.
But UK/LHR is Included.
Because I booked for my sister LHR-PHX starting at LHR, and I used $300 of My trip credits for cancelled US domestic flights.