Where the source texture is virtually located, can be defined with the Source Location controller. Image 3 for instance has the source texture (the flower) located at the right side, Image 2 at the top.

How much of the target texture is replaced by the source texture can be regulated with the Border parameters. The Width value controls the percentage, if the source texture is located at the left or right side. Height is for source textures, which sit above or below the target texture.