Usually one will remove a device if passed on or sold to someone else. If it stop working and/or you are done using it but will keep it in a drawer somewhere one would retire the device. In either case, you do not use any of the activities associated with the device.
I am unable to figure out how to re-add a device I retired. Anyone have the answer? the 910 can be recharged while running and need it for my upcoming race. My current 920 can not so not planning on using it.
You have to re-add it via Garmin Express. If it's still there, simply remove it and add again, Express will ask you to log in to your Garmin Connect account and the device should then appear there.