Why PSK31 software (and the common IMD meters built into digital mode programs) measures intermodulation products of an idling signal relative to the main tone pair, and the number is negative because the IMD products must sit below the desired signal. A level of -30 dB or lower means the distortion products are at least 30 dB down, which is the generally accepted threshold for a clean PSK signal. If you drive the transmitter too hard, the ALC acts and the IMD figure climbs toward -20 dB or worse, splattering into adjacent frequencies.
Watch out The positive values are physically nonsensical here, since they would mean the distortion products are stronger than the signal itself; they are just there to catch someone who ignores the sign.
IMD is measured downward: only the negative number can be right, and -30 dB is the clean-signal target.