The zero, the + sign and the - sign relate to what we call "Unity Gain". This comes from the analog mixer days, so to speak but is also related to electrical engineering standards that we don't care to know too much about for the sake of understanding what it means on the Volume Unit meter.
Basically the "0" on the fader is unity gain. Unity Gain means that there is "essentially or relatively speaking" no increase or decrease to the signal as it passes through the faders' gain altering electrical components. You can think of each fader on a mixer console as mini amplifiers. A given signal level in the circuit before the fader with the fader at "0" would give you no change in the signal level. Raising and lowering the fader simply amplifies or attenuates the signal level. The + numbers and - numbers are more for reference. I think the term "decibel" is used loosely in this context because the actual decibel change is relative to sound pressure, air movement, loudness and how the human ear typically perceives volume and loudness. Decibel is a relative measurement the same way inches, feet or miles are. If I ask you the place my desk at 6 inches you will want to know where to begin measuring from and might assume I mean 6 inches from the wall. If I tell you a gas station is 3 miles down the road you are automatically using a reference point from the location where we are holding the discussion.
If you tell me to turn up the music to 80 db I am making an assumption that our starting point is at the lowest level that we can perceive in the current room we are in. I don't really measure the audible sound in the room. I make assumtions or use a Sound Pressure Level meter. But, I already know that 80 db is typically the level of a 2 person conversation. Not very loud, but comfortable.
Decibels, for example; a 6 db increase is commonly perceived as twice as loud as what ever the current decibel level is. We use a Sound Pressure Level meter that puts numbers on the sound pressure so we can compare and evaluate it. The music is playing at around 85 db as read on a SPL meter at the front row of an empty auditorium. We might suggest leaving enough room on the fader so that we can raise it 6 to 10 db. So, if the fader is at zero we lower the fader - 10 db (referenced on the fader) and turn up the main amplifiers to bring the music back to 85 db. When the audience shows up we've got plenty of lee-way on the console without messing with the amps.
Anyway, that's a quick analog analogy.
The software DAW is borrowing common thread themes from the hardware days. In recording and mixing in a software DAW we use the VU meter and the faders' db scale as a quick and easy reference. I know that if I have 2 - 3 guitar tracks with the faders at zero and my main output VU meter is already hitting zero also then as I record more instrument tracks my main VU meter goes red (clips) and the sound is distorted. With some experience I have learned to start out with my faders a bit lower depending on how many tracks I intend to record and mix. The VU meter and the numbers on it are there to give me a visual to relate to.
Well, I'm a bit tired so I hope this helps you understand a little more.

Last edited by Tobias; 01/14/17 09:02 PM.
Does the noise in your head bother me ?