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this is only one measure in the mathematical sense.
a minimum open cover of a set is not the only measure -- but maybe easiest way to understand. like tiling a bathroom floor; infinite number of ways to cut pieces of tile, but one total area of the floor: the area is the 'measure' of the floor. this is just one basic, intuitive measure; by no means the only one, and not strictly the definition of 'measure' but an example of one.
ex. you can define a measure as the smallest number of tiles you can use, or the smallest total length of grout between all the individual tiles.