Avidyne IFD540 Pilot's Manual page 161

Nav Source
State
GPS
GPS VLOC

VLOC
OBS
5-5 Navigation
GPS is the active state, and all external
deviation data being transmitted by the IFD is
in reference to the active GPS leg. Note that
if no GPS leg is active or if no GPS lock, the
nav mode will be displayed in yellow.
GPS is the active state, and VLOC is armed.
If the GPS flight plan contains a VHF-based
procedure or leg (e.g. localizer final), then the
nav mode will automatically transition from
GPS to VLOC when the capture criteria have
been met. While GPS is active, all external
deviation data being transmitted by the IFD is
in reference to the active GPS leg. That
transitions to VHF deviation data when the
switch to VLOC is made.
VLOC is the active state, and all external
deviation data being transmitted by the IFD is
in reference to the active VHF-based leg (e.g.
Inbound VOR course, localizer deviation,
etc). Note that if no VHF data is being
received (e.g. out of range, improperly tuned
nav radio, etc), the nav mode will be
displayed in yellow.
OBS becomes the active state and deviation
data being transmitted by the IFD is reactive
to the external course setting knob (or the
Nav Source knob if no external course control
is present) for the inbound/outbound course.
Note that this state is accessed by pushing in
the Nav Source knob, and then it takes the
place of pure GPS
in this table.
Description
mode as described earlier