Decals are basically PBR Materials which can be projected directly onto the scene geometry. This can be used to quickly place signs, logos, dirt or other effects without complex UV mapping of 3D models.
A decal object works like an orthographic projector, which affects all objects within its selected area and along the Y+ axis.
Add a decal from the library
Drag the decal from the library and drop it over the object you want:
Drag a Decal from library
Drop to the object surface
Using the orange handles, you can scale the decal. Hold SHIFT to scale proportional.
Scale a decal
Move and Rotate
As for any object, you can move/rotate the decal on the surface object:
Move a decal
Rotate a decal
Create a custom decal
To create a new decal, add a new decal from menu Create > 3D Objects > Decal
Add a decal
Add your image as a Diffuse Texture
You can save this custom decal in My Library for easy reuse!