Mixing boards generally have three kinds of outputs:
To spatialize sound among more than two speakers, you need as many independant mixing board outputs as you have speakers, so generally the main outputs are not useful.
Most mixing boards have individual knobs (or "pots": "potentiometers") on each channel to control the level of that channel to the auxiliary sends. Some auxiliary sends are "pre-fader", meaning that the level sent to an auxiliary mix from a channel does not depend on the position of the fader on that channel. Other auxiliary sends are "post-fader," meaning that the amount of signal sent from a given channel to a given aux send is determined both by the position of the knob and by the position of the fader. Each is useful in certain situations; on some mixing boards there is a way to select whether aux sends will be pre-fader or post fader.
This gives extremely flexible routing of mixer inputs to the various speakers. Each mixer input can be sent in any amount to any of the outputs; this is the "general case" of maximum control of the mix.
All mixing boards that I know of have an even number of sub mixes (zero being an even number...). Mixing boards with sub mixes group them concepually into pairs 1-2, 3-4, 5-6, 7-8; in many ways these pairs are equivalent to the left-right pair that constitute the main stereo output of the board.
Near the fader for each channel is a set of push buttons with labels like "L-R", "1-2", "3-4", etc. When this button is in, the signal from the channel is sent to the given two busses. Any number of buttons may be in or out, so channel 3 could be sent to the LR mix and groups 3 and 4, while channel 4 is sent just to groups 7 and 8.
Each channel has a pan knob, generally directly above the fader, that determines the relative level of the signal in each of the pairs L-R, 1-2, 3-4, etc. So if channel 3 has the "L-R" and "3-4" buttons pushed in and the knob is 90% to the right, channel 3 will be sent a little bit to the L mix, a lot to the R mix, a little bit to the 3 mix, and a lot to the 4 mix.
Thus, the control of how much a given channel goes to each of the sub mixes is not fully independent. There is generally no way to determine how much of a signal goes to a sub, except for all-or-nothing selection of whether the button is in, and the 1-dimensional control between pairs of inputs given by the position of the single pan knob.