A zigbee thing I've always wondered about but never took the time to ask.....
Anyone who has added a zigbee device to HE is familiar with the warning about perhaps needing to "factory reset" the device to ensure its discoverable. Check.
At the same time, I've heard advice around these parts that, if you have a zigbee device that's fallen off the mesh, you can just re-join the device. It will automatically inherit the name, the associations, and the apps that the device had previously. Cool trick. Check.
But how do we reconcile #1 and #2? They seem inconsistent to this noob. I'm sure there's a perfectly good explanation. I just don't know what it is.
I'm not a specialist by any stretch of the imagination - When I pair a device - it id's itself using a DNI... a sort of MAC address. I believe thats specific to the device. Since that's the case, HE can see the device was previously on the network and offers to put it back. I've factory reset bulbs because HE stops talking (or they had stopped listening), and HE still remembers who they are.
What is more questionable is if a device is removed from your network physically, but your device is left behind in HE... I'm actually cool with that. there should be a 14 day grace or something. Device Watchdog app does help show devices that haven't been communicated to in a while. There's also drivers out there that 'ping' (ohlalalabs drivers EG) that can help identify lost goods.
#1 applies mostly to device that are totally new to that hub. For Zigbee devices, normally a reset is also how they get put in "pairing mode" (a bit different from Z-Wave, where these are normally separate processes, though I don't doubt that there may be some odd), though some brand-new devices might also do that on first power-up. Doesn't hurt either way. But it's not normally necessary if the device just "falls off" your network, say because the battery went dead. In that case, just replacing the battery--without resetting/re-pairing the device or putting Hubitat in Zigbee discovery mode--should suffice.
#2 is really only applicable if you do need to reset/re-pair the device for some reason. This could be if the above (e.g., just replacing the battery) happens to fail for some reason, or maybe you moved your backup to a new hub (one of the cool things about Zigbee is that one of the identifting pieces of infomation the hub/coordaintor uses is, indeed, the device's factory-set MAC address, so they can be matched up even if you do that). This is different from Z-Wave, where the hub/controller only cares about the node ID, which is normally assigned at pairing (and if you mess up, or don't catch that the hub sort of did, maybe because of a failed pairing, it's possible to have the same node assigned two IDs, only one of which will work).
But...if your device was previously paired to the same network, then usually neither of the above comes into play.
AFAIK, all Zigbee devices are essentially 'factory reset' by the same action that initiates the pairing process. Putting a Zigbee device into join mode is equivalent to factory resetting it.
Not so with Z-Wave; if you try to include or exclude an early generation GE/Jasco dimmer you find that there may be a sequence of paddle taps to factory reset it vs. a single tap to initiate exclusion or inclusion, depending on the hub's mode-- and some of the earliest devices (the GE 12722 switch is one example) have no local reset capability at all-- they require exclusion to be initiated by a Z-Wave controller before they can be included in another network. It was only with the debut of Z-Wave Plus that local device reset capability was mandated for all devices.
I suspect that this early Z-Wave design choice (not providing local reset capability) was intended to require the involvement of the controller when removing a device from the network; it's essential to Z-Wave routing that the controller always have an accurate inventory of devices or it will generate routes through devices that no longer exist (a symptom of the Z-Wave ghost problem). Zigbee, on the other hand, tolerates routing devices that suddenly disappear; Zigbee routers always maintain awareness of their current neighbors without any intervention from the hub.