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.
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Type
Toggle the emitter's type with this drop-down menu. The following parameters and the viewport representation will be updated according to your selection. Bear in mind that type changes will change the emitter's and fluid's behaviour completely.
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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 triangle's inner area remains empty.
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