The dB value of each frequency range represents the average dB level measured within the respective range. The exact value is only relevant, if you want to use true:balance not only to measure the overall spectral shape or balance, but also the absolute signal level in a certain frequency ranges.
To set an absolute level reference, you can disable the "follow-mode" of the reference zone (click the small "A" on the left hand side of the reference zone). Once disabled, you can set a fixed level for the zone and compare a track against this absolute reference.