The source texture is either chosen by enabling thebutton on its form, or by selecting it at the Source Texture panel. Move the source texture by manipulating the Offset values. You can always see the source texture in relation to your target texture by configuring the viewport with the button and enabling thebutton on the target texture. This causes the source texture to be displayed as an overlay on the target texture.

Target and source texture do not always have the same width and height. While the Tile option takes the source texture unchanged, the Adjust switch causes it to be scaled to fit the target texture's dimensions.