How is a Part's Postion and Size substracted visually to get the edge for Region 3 formula?

Q: How is substracting a parts position from its size to get the outermost bottom edge seen visually?

E: In my case I am trying to get the outermost bottom edge for my region 3 formula for position 1, I am confused at the last step: where you substract the parts position from the parts size divided by 2.

How does that shrink it all the way down to the outermost bottom edge visually?

Here is an example I made explaining in further depth:

Region 3 Formula for Position 1: We want the bottom outermost edge.

local pos1 = regionpart.Position - (regionpart.Size/2)

Region3 is deprecated, probably better off using something else like Spatial Query