How do you decal properly?

[voon]

To keep it simple: Some cube, with some metal material whatever with its own texture etc.

Now how do I add a non-repeating single decal anywhere onto it? Some little sign etc out of a decal texture with lots of decals on it.

[FrankPooleFloating]

Keeping it simple: Use Mix Material and plug a b/w image (opacity map) into amount. If a UV were present in this cube.obj, you would want them (decal & white) placed in uv maps where they should appear in UV space. Alternatively, you could use projection node (box) and place and scale as needed.