The Usage flags are used to control the types of objects on which the Material will be used. When compiling Materials, these settings allow the Engine to compile special versions for each application. These are only valid when using the Surface Material Domain setting.
In the editor, these flags will be set automatically for any objects that already exist within your maps. For instance, if you have a Particle System that uses your Material placed somewhere in a level, when you load that map in the editor, it will automatically set the Used with Particle System flag. The material asset will need to be saved to allow the game to use the material on that specific mesh.
If you do not have the appropriate usage flag set, the default world grid material will be used in-game instead! This will have an appropriate message in the log of the game client.
Used with Skeletal Mesh
Set this if the Material will be placed on a Static Mesh.
Used with Editor Compositing
Set if the Material will be used with the Editor UI.
Used with Landscape
Set if the Material will be used on a Landscape surface.
Used with Particle Sprites
Used if this Material will be placed on a Particle System.
Used with Beam Trails
Set if the Material will be used with Beam Trails.
Used with Mesh Particles
Indicates that the Material and its instances can be used with mesh particles. This will result in the shaders required to support mesh particles being compiled which will increase shader compile time and memory usage.
Used with Static Lighting
This is set if the Material will be considered for Static Lighting, such as if it uses an Emissive effect that should affect lighting.
Used with Fluid Surfaces
Fluid surfaces are no longer supported in Unreal Engine 4. This option will soon be removed.
Used with Morph Targets
Set if this Material will be applied to a Skeletal Mesh that utilizes a Morph Target.
Used with Spline Meshes
Set if the Material will be applied to Landscape Spline meshes.
Used with Instanced Static Meshes
Set if the Material is intended to be applied to Instanced Static Meshes.
Used with Distortion
Distortion is no longer supported (it is now Refraction) and this option will soon be removed.
Used with Clothing
This should be set if the Material will be applied to Apex physically simulated clothing.
Used with UI
This indicates that this Material and any Material Instances can be used with Slate UI and UMG.
Automatically Set Usage in Editor
Whether to automatically set usage flags based on what the Material is applied to in the Editor. The default option for this is enabled.