In most software, a tick box either selects or deselects a particular option: simple. So you'd expect a group with no boxes ticked to be equivalent to selecting none of the options, and you tick one or more of them to add the options you want. In fact, no ticks seems to be the same as all-ticks: ticking a single box in an unticked group surprisingly and without notice deselects all the options other than the one selected.
Now it's true that this is documented half way down the long documentation page, but it should be clear on the face of the controls themselves.