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- Create a standard material and add a bitmap or a shader like noise to one of the channels, e.g. "Color".
- Drag the material's texture thumbnail to the "Texture" slot of the parameter.
- White parts of the image/shader will be used to create the particles.
- Reset, if necessary.
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Here's an image of a capsule with Cinema 4D's standard noise shader used as a mask for the particle layer:
Image capsule with noise-pattern-based particle layer