It's a dummy button.
Regarding the sound quality. It might just be in my head but I'm fairly certain this sounds better than my previous unit. Before anything around 32/33 on the volume dial was hit or miss. Now I can hit 34/35 and it sounds great. Although, it's entirely possible that the scale is just not 100% in sync w/ other units. FM channels sound great now since the ones I listen to locally all have HD feeds they're sending through.
I wish Subaru would post the wattage of the units. This radio is paired w/ a 6-speaker setup while my old unit on the Forester is w/ 4 speakers. I'm curious if they designed it to have a few more watts because 1. it's a higher grade radio and 2. it's paired w/ more speakers. Who knows, pure speculation. I'm going to pull the unit out again this week to install my sub so I'll take some detailed pics of the rear and the product information stickers on the top.