Collision Modifier
The Collision modifier is a container for a Collision Physics. Collision physics provide interaction between different physics simulations.
Options
As the modifier is only a container its actual options can be configured in the Physics Properties tab. See the Collision Physics Properties for more information.
Example
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Deflected particles.
Here is a Meta object, using Instancing Vertices to a particle system emitting downwards, and deflected by a mesh cube.