Most Apogee products that use the Apogee Maestro 2 control application will include a 'Device ID' button that can be found in multiple Maestro tabs under a graphical icon for the connected Apogee device. You will usually see this button as an 'A'.
Here is an example of what it looks like when you have a Symphony I/O connected:
The purpose of this button is to quickly identify which Apogee device you're adjusting parameters in Maestro for. When you click on the button, it will light up the front-panel of the corresponding Apogee device so you can see which device you are adjusting parameters in Maestro for. Click the button again and the device will no longer be identified.
Since multiple Apogee devices can be connected and appear in Maestro at the same time, it can sometimes be confusing to know which device you're adjusting parameters for. Having more than one Duet or Quartet connected to a Mac may not be as common as having multiple Symphony I/Os or Ensembles connected, but the 'Device ID' buttons are there for each product, just in case.
A typical use-case:
You have 2 Symphony I/Os connected to one Mac and both are showing up in Apogee Maestro. You can go to the 'Device Settings' tab in Maestro and change the peripheral prefix of the 2nd Symphony I/O to 'B' (or any other letter you want). Now, when you click the 'B' button (Device ID) in Maestro, the 2nd Symphony I/O will become identified. If you click the 'A' button, the 1st Symphony I/O will become identified.