When I tried to use the dx, dy and dz selectors for defining an area, the area was not correct.
Explanation:
command/selectors: /say @a[x=2002,y=26,z=2000,dx=1992,dy=27,dz=2009]
mostly the feedback was: "That Player could not be found"
except when I was standing in the area from [x=2002,y=26,z=2009] to [dx=posX*,dy=posY*,dz=posZ*] (The posX* means
"infinitely Towards Positive X", The posY means "infinitely Towards Positive Y" and the posZ means "infinitely Towards Positive Z"
So I can stand anywhere in the south east above Y=26 from [x=2002,z=2000].