+4 and -10 refer to operating levels. +4 is considered "pro" and -10 is often considered consumer or “line” level. An operating level indicates what a zero reading on a meter equals. A device set up to operate at -10, will show a reading of zero on a VU meter; this indicates an output level of -10 db. A +4 system reading zero on a VU 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 on the inserts of a channel of a mixer, the usual level is -10. You should check the manual of the device with which you are using your ACP88 to determine the zero operating level of the device and adjust the ACP88 accordingly. If you are unsure, the -10 setting on the ACP88 will output and handle lower level signals than the unit would were it set to operate at +4.