No matter 0dB, -18dB or +8dB , all is a reference.
In digital audio, 0dB is the max level, so we set the
referencr lower than 0dB, e.g. - 12db , or even -24dB for
classical music ,just depand on the dynamic range of the
programme u record.
In Analog audio, we "usually" set the reference as 0dB,
but not "must",
JUST BECAUSE most of the home recording equipment is
not professional enough,
i.e. not enough headroom, so it may need different "reference"
ALSO, 用mic preamp经A/D设备直接输入电脑，也是遵循这个原理 .
BTW, no matter 数字设备的dB 和 模拟设备的dB , THEY ARE THE SAME