Sorry, maybe a bit more clarification is needed.
I have never seen a TCAS display that could be switched to show flight levels. My reference to flight levels was about what ATC would see on their radar display. ATC will see the actual altitudes of the aircraft, but the aircraft themselves will only see the altitude difference.
I should have been more precise about the arrows: It will be an up arrow if the traffic is climbing at a rate of 500fpm or more; it will be a down arrow if the traffic is descending 500fpm or more.
DD

edited cause I cant spel proply

Desert Dingo

Our TCAS will show the flight level of other traffic (rather than the relative level) if the F/L switch on the transponder is pressed. This is a momentary action switch, so it has to be held down, but it is rarely used.

G W-H

Thanks G W-H, I did not know about that.
Advancing technology has left me behind.
Again :{