For more info about the below, please see the Unreal Engine documentation page.
The Dynamic module is used to pass four scalar values to the material(s) used by the emitter allowing the emitter to control material effects.
You may use particle color to set a material particle to emit a specific color and opacity as determined in the cascade particle editor.
Alternatively, Particle Color can also be used to drive other effects so long as each channel is used in a similar manner (if one channel is static, the others should also be static). An example of this usage is adding heat distortion to a particle effect.
The Red channel controls the emissive color, the green channel controls the normals scale, the blue channel controls the camera distance fade and the alpha channel controls the strength of refraction (distortion). Courtesy Richard Stokes.
The ParticleDirection expression outputs Vector3 (RGB) data on a per-particle basis, representing the direction a given particle is currently traveling.
The ParticleMacroUV expression outputs UV texture coordinates that can be used to map any 2d texture onto the entire particle system in a continuous way, meaning the texture will be seamless across particles. The UVs will be centered around MacroUVPosition (specified in Cascade on the ParticleSystem, under the MacroUV category) and MacroUVRadius determines the world space radius that the UVs should tile at. The ParticleMacroUV node is useful for mapping continuous noise onto particles to break up the pattern introduced by mapping a texture onto each particle with normal texture coordinate
