Garmin G3X Touch Pilot's Manual page 252

AFCS
As the aircraft nears the VNAV Target Altitude, AFCS automatically transitions
to VNAV Target Altitude Capture Mode with Altitude Hold Mode armed. This
automatic transition is indicated by the green 'ALTV' annunciation flashing for
up to 10 seconds and the appearance of the white 'ALT' annunciation.
At 50 feet from the VNAV Target Altitude, the AFCS automatically transitions from
VNAV Target Altitude Capture to Altitude Hold Mode and selects the VNAV target
altitude as the new Autopilot Altitude Reference. As Altitude Hold Mode becomes
active, the white 'ALT' annunciation moves to the active vertical mode field and flashes
green for 10 seconds to indicate the automatic transition.
Automatic Mode Transitions During Altitude Capture
234
Automatic Transition to ALTV
Mode with ALT Mode Armed
Automatic Transition
from ALTV to ALT Mode
Garmin G3X Touch Pilot's Guide
Altitude
Reference
190-01754-00 Rev. A