Editing Main Render Settings (section)

=== Decal Positioning / Sizing ===
The decal UV coordinates specify the centre of the decal. The centre of the decal remains fixed when rotation and scaling are applied.

Although the decal's position can be set manually, it is much easier to zoom-in to where you want the decal to be, then click the "Click here to set decal position" button. Note that this uses Unity's raycasting, which does not always work reliably (I'm not sure why). If it does not work, move or rotate the camera slightly and try again.

If tiling is on, the decal will repeat. Otherwise it will only appear in one location.