When Focus is at 0 the central Boost knob does correspond to a between 0dB and 12dB increase, HOWEVER, as soon as you move focus this changes the amount of boost so that more is applied to quieter passages. At max focus, Boost is designed so that it never hits the limiter, instead applying 0dB gain to values that are already at 0dBFS. If the input (after the pre-control) is greater than 0dBFS then the compressor / limiter will always apply -ve gain to keep it under 0dBFS (e.g. the limiter kicks in). The above is a simplified explanation of Peak Remapping technology this along with the behaviour of the controls are also mentioned in the manual.

If you want to specify an absolute gain in dB, the input gain slider on the left can achieve this. With the Boost knob and Focus at 0 the peak limiter will still activate to keep the signal below 0dBFS.