Garmin GTN XI Series Manual page 34

Before
If you initiate a transfer before completing frequency entry, the direct tuning field
autofills the remaining characters, enters the frequency into the standby field, then
swaps it with the active frequency.
After
Tapping this window swaps the active frequency value with
the standby frequency displayed in the lower window.
Tap once to swap the displayed frequency values. Tap again
to swap them back.
Tapping this key on the COM Standby control panel
performs the same function as tapping the COM active
frequency window.
Pushing and holding the control knob for 0.5 seconds
automatically flip-flops the active and standby frequency
values.
annunciator bar
Standby and active COM frequency values swap
The transfer (or flip-flop)
function allows you to swap
the active and standby
frequency values.
This function is accessible
multiple ways.
A transfer icon indicates that flip-flop
functionality is available.