+4 and -10 refer to operating levels. +4 is considered "pro" and -10 is often considered consumer level. An operating level indicates what a zero reading on a meter equals. On a VU meter that is on a device set up to operate at -10, a reading of zero on the meter indicates an output level of -10 db. A +4 system at zero on the meter is actually putting out 4 db of signal. In a +4 system, .775 millivolts across 1 watt causes a reading of zero on the meter. This is how the meter is calibrated. If you are using the ACP at the inserts of a channel of a mixer, the usual level is -10. You should check the manual of the device you are using your ACP 88 with to ascertain the o-operating level of the device and adjust the 88, accordingly. If you are unsure, the -10 setting on the 88 outputs and handles lower level signals than the unit would were it set to operate at +4.