Each channel of the mixing console posesses a panpot (short for 'panoramic potentiometer').
In early mixing consoles the signal could be switched to the left channel, to the right channel, or divided equally between left and right - center, in other words.
The panpot can be used to send the signal to any point in the stereo image between extreme left and extreme right - known as 'hard left' and 'hard right'.
The panpot has what is known as a 'law'. This determines the way the signal is proportioned according to the position of the panpot.
In some mixing consoles, when the panpot is in its center position, the signal level is -6 dB compared to hard left or right. This is because the signal is routed equally to left and right and the two sides add together.
Other consoles use a -3 dB law.
The object is to maintain a constant signal level no matter where the signal is panned.
-6 dB is theoretically correct if the stereo signal is later going to be summed into mono. -3 dB is correct if the signal is to remain in stereo. Some consoles use a compromise -4.5 dB law.
Most engineers would regard this as a fine distinction, hardly worth bothering about.