This is an area emitter. An arrow indicates the node’s emission direction and helps you to point the emitter to a certain target. You can position (W key), rotate (E), and scale (R) this emitter.
Volume
This feature creates a defined volume of particles. The size of this volume is defined by the emitter’s horizontal → “Scale” “Scale” settings and the vertical 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.
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Here the particles' initial speed is defined in metres per second. Higher values will generate more particles per frame. Faster particles act with stronger forces on other particles or objects. The number of emitted particles also depends on your → “Resolution” “Resolution” settings. A value of 0.0 disables the emission of particles.
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Normally the particles are poured out along the emission axis, but it possible to emit the particles from the emitter's edges. Then, the circle’s square's inner area remains empty.