Minecraft wiki entry for testforblocks, associated text: Specifies two opposing corners of the region to use as the pattern to test for (the "source region").
It must be a three-dimensional coordinates composed of <x>, <y> and <z>, each of which must be a floating-point number or tilde and caret notation.
Block position is the coordinates of the point at the lower northwest corner of a block. Because of this, the lesser coordinates of each axis falls right on the region boundary, but the greater coordinates are one block from the boundary, and the block volume of the source region is (xgreater - xlesser + 1) × (ygreater - ylesser + 1) × (zgreater - zlesser + 1). For example, 0 0 0 0 0 0 has a 1-block volume, and 0 0 0 1 1 1 and 1 1 1 0 0 0 both identify the same region with an 8-block volume.