Unless I’m mistaken, this is probably a typo, as the standard is +4 dBu and -10 dBV. The simplest means of explaining what this means comes courtesy of GearSlutz:
The difference between +4dBu and -10dBV [commonly shortended to “+4” and “-10”] is about 11.79dB [the difference is based on a logarithmic scale so its not a straight “14 db” as you might think]… what complicates it further is that you’re dealing with two different scales [dBu being “dB - un weighted” and dbV - dbVolts"] both with a reference to 0db = .775volts… dBu can be used regardless of impedance, but is derived from a 600 Ω load dissipating 0 dBm (1 mW).
When you set the input level to “-10” you’re increasing the input gain [by 11.79 db]… which originally done to reduce the noise level in “semi pro” equipment [hotter signal in, less noise… also less headroom and a whole bunch of other issues but I have a feeling I’ve already gone a little too far with the explanation].
All you need to know is
If you plug a +4dBu output into a -10dBV input the signal is coming in 11.79dB hotter than the gear was designed for… turn something down.
If you plug a -10dBV output into a +4dBu input the signal is coming in 11.79dB quieter than the gear was designed for… turn something up.