The Mirror app is a simple app designed to send commands to one or more "replica" (secondary) devices based on events from a "primary" device.
NOTE: The "mirroring" is one-way, from the primary device to the replica device(s). Only changes registered on the primary device will cause commands to be sent to the replica devices. Changing the replica devices will not cause any changes elsewhere (nor will running commands on the primary device that do not actually cause state changes or that do not correspond to one of the events listed as "forwarded" in the interface after all the devices are selected).
The Mirror app offers several different types of "child apps," depending on what your goals are.
The "Device Mirror" type is intended for bulbs, dimmers, switches, and similar devices. Commands for switch, level, and color/color temperature will be sent to replica devices based on events generated on the primary device. It was specifically designed for use with certain remote controls that have color wheels or slider-type surfaces that internally track their state (the intent being that this app is used to mirror those states to bulbs, dimmers, etc.), but it can be used for any case where you want one or more devices to "follow" the reported state of another.
Select Create New Device Mirror... to create a Device Mirror setup (these child apps will be called "Mirror Me"), then configure it as desired. For example:
Select Create New Lock Mirror... to create a Lock Mirror setup to make one or more locks follow one or more other locks. This type of mirror is intended to send "Lock" or "Unlock" commands to the replica lock(s) based on events generated from the source lock(s) — e.g., when a source lock is unlocked and reports that event to the hub, an "Unlock" command will be sent to the replica locks, and similar with the "Lock" command.
lockattribute you can see under "Current States" on the device detail page).
Select Create New Media Input Source Mirror... to create a Media Input Source mirror setup to make one or more devices that support the "Set Media Input" command follow another such device. When the media source attribute on the source device changes, the replica device(s) will be sent a corresponding "Set Media Input" command to match. Optionally, additional commands can be sent to replica devices based on source device events, as described below.
Select Create New Custom Mirror... to create a custom "mapping" from source device attributes to destination device commands. For example, a Custom Mirror can be used to:
Different "capability groups" are offered, and this is the first selection you will need to make when creating a custom mirror. The "Switch Level, Window Shade, Audio Volume" group type can be used to map a range of values, usually 0-100, to commands/attributes that also accept/report values in that range. The other group type can map any of several discrete values the device reports (e.g., motion active or inactive) to another set of discrete values (e.g., contact open or closed, presence present or not present, etc.).