Intended for riders who already own an eTap equipped bike and want to use a cassette with more than 28 teeth. Lets you go to 32 teeth.  Just took this off I bike that I had upgraded to Red AXS.  Fully functional. I had someone return the item because they could not get it paired.  When I got it back I was able to pair it with both SRAM Red ETap Shifters and Red AXS Shifters.  All functions worked correctly. No battery included.

 Please feel free to ask questions.