Mix Level on Channels in Audio Ports can be Negative

Negative percentages seem to be allowed on the Mix Level of a channel on an Audio Input Port. Does that have meaning?

My rig is broken down right now for transport, so I can’t test for a few days … but I was wondering if this is intended (and what it does - invert the Waveform? Suck Sound from other places in your system?) or maybe it’s an oversight?