From BALERT
Just had a look at Tiger web-site. They are using code "TTnnn" within Australia but "TRnnn" on their International flights. Why on earth would they be doing this?
Just a wild guess BA but I would guess they are attempting to differentiate between internal and international flights.
To some people BA these aircraft all look the same.
Maybe to stop simple people like someone we know from attempting to catch the wrong flight.....