Decals are Materials that are projected onto meshes in your level, including Static Meshes and Skeletal Meshes. These meshes can have a Mobility setting of Static or Movable and the Decal will still project on them. Many Decals can be rendered at once without a large performance decrease. Performance decreases with larger screen space size and higher shader instruction count.
You can add the Deferred Decal Actor directly from the All Classes tab of the Modes panel. You then need to make a new Material and change the Material Domain property to Deferred Decal in the Material's Details panel. You can then apply the Material to the Decal Actor.
The Decal Actor is displayed as a wireframe box. There is an arrow always in the center that points in the direction of the decal projection.
The distance of projection for a decal is determined by the X-Axis scale of the box surrounding the decal sprite. As surfaces approach the extent of this box, there is a blending where the decal fades away.
In the image above, you can see the decal projection on the back wall starting to blend away as the surface gets too far from the Decal Actor.