“Circle” is an area emitter. Emitters define the point of particle creation in 3D space, while the particles are part of the "Fluid" container. Emitters can be scaled, rotated, and translated like objects; an arrow indicates the direction of emission.
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This feature creates a defined volume of particles. The size of this volume is defined by the emission area and the value entered here. All dimensions are given in metres. By activating this option, the emitter’s speed value will be set to 0.0 automatically The parameter is connected to Cinema 4D's scale settings.
Speed
Here the particles' initial speed is defined in metres per second. Higher values will generate more particles per frame. The number of emitted particles also depends on the domain's “Resolution” settings. A value of 0.0 stops the emission of particles. The parameter is connected to Cinema 4D's scale settings.
Vertical Random
To avoid regular emission, it is possible to randomly displace the particles with these functions. The value's range is between 0 and 1.
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You are able to define a ring from where the particles will be emitted. Small values create a larger emission area, higher settings are responsible for thin rings. The parameter ranges from 0 to 1.