You're not reading it right. The bottom label says 6 feef. Clearly this is some sort of measurement of 4th dimensional movement that accounts for the other measurements.
Minimum. This is obviously saying keep a minimum of 6-feet of distance in each direction. Who would ever recommend either an exact distance or a maximum safe distance!?
A cube needs 3 coordinates, 0.0.0 to one corner, 1.1.1 to the opposite one. And its size is square root of 3 (if the diagonal of a 2d square is square root of 2, and the one of the cube is square root of 3, can be said that the diagonal of a n dimension cube is square root of n?)