When you look through the view finder, the array of focus points is a matrix of small squares. But each of those small squares also has an even smaller square in the center of it. When using Single Point AF, you are using the larger, outer square for that point. When using Single Point Spot AF, you are using the tiny square in the center of each focus point.
I had assumed that when just the central spot has the dot in it, the single point spot AF was activated for that central spot. But when each of the array of squares has has a dot, it is an indication that the whole of that array is active, and not the centre of each component.
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