The Sphere shader basically does as advertised: renders particles as spheres. This is a good starting point for rendering typical effects like bubbles to billowing smoke or even lava spewing from a volcano.
The Face parameter lets you render both faces of the particle or just the front or back. You can select a Texture Projection style (cylindrical, lollipop, etc.); and if the particle types have 3D rotations on them, you can choose to render the rotations.
For more details on each of the parameters, see Particle Sphere [Shader Reference].
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