The only mention of it in the manual is on page 37:
'When this unit detects Dolby Digital signals, the following display appears for a few seconds. This shows how the signal is being corrected to become -27dB (THX recommendation)'
If it shows DialNorm: +4dB that means the DialNorm value in the bitstream said the average dialog level was -31dB and it boosted it 4dB to make it -27. If it doesn't appear then either the level was already -27dB or the bitstream didn't have a DialNorm value. The DialNorm value is optional and not always present, let alone accurate when it is present.
The Dolby site says the recommended level is -31dB. I didn't know the THX recommendation is -27dB but if Yamaha says so, it must be true.