Need info re. "lifeguard"

[parks]

Can anyone advise what "lifeguard" refers to in the context of RT use after flight-number .......... e.g. TWA 800 lifeguard?

[411A]

Medevac...commonly used in the USA....although normally used before the flight number/aircraft registration.

Also, used when human organs onboard, for transplant.

[quid]

.....and alerts ATC so there won't be any unnecessary delays. The other guy gets vectored first.