What operating system are you referring to?  Mac Windows or mobile?

The settings slider on the app

PLEASE be more specific. Maybe you could post a screenshot?

I think @PaulinBont might be referring to the Sub or Surround settings.

Positive values (0 → 15) increase the amount of sound that come from that product and negative values (-15 → 0) decrease the amount of sound that comes from that product.

Increase it if you want your sub/surrounds to be louder, decrease if it’s too much 

Thanks James,

I get that but why the negative values, that’s my point. If you want to decrease volume surely 15 down to zero works  to reduce decibel levels but why negative values from -15 up to zero?

Because 0 is the norm.  positive values are above the norm, negative below. 

0 is the neutral, default setting. You can boost or reduce from that according to preference. 

The rationale behind that is that 0 is the middle point; the default. If you want more than that, increase above 0, if you want less, you go below 0. These settings were implemented long before I started working here, so I can’t comment on the exact design choices, but I’m almost positive we’re not the only one doing things this way  It’s quite common to use negative values in audio production as well as some home audio products.

 

Edit: Looks like I was too slow typing my response! Thanks jgatie and nik9669a 

Thank you for explaining that one to me nik; I get it now: -1 is one increment quieter than neutral/default and +2  is three increments louder than that, ( or two increments older than default/neutral).

My old amp simply went 1 up to 10

The answers above have made me think of another question:

Because the Sub relieves the Arc of doing the lower frequency work, does that mean that I should set the Arc bass value to-15 or does it mean that the Sub needs an optimal Arc setting to do what it does best?

The Sonos system adjust those settings for you, depending on whether or not you have a Sub bonded. When you add a Sub, there’s no need to adjust the settings, unless you feel like you need to, when hearing the system. It’s all relative to your ears, and your expectations, there’s no need to adjust the settings when you add a new device, unless your ears tell you you need to. 

Spinal Tap had amps with volume going up to 11; think what volume you can get now that you have one going to 15!!!