By default, the emitter is set as a Rectangle.
The shape can be changed, by using the drop-down box, to: Circle, Ellipse, Sphere, Box, Cylinder or Torus.
The particles can also be emitted from a defined Object, a Defined Emission, or an Instance.
The settings below change dependent on the Emitter Shape selected.
This image shows the 2D Emitter Shape settings of Rectangle, Circle and Ellipse.
Here, you can see the 3D Emitter Shape settings of Sphere, Box, Cylinder and Torus.
Image showing the Emitter Shape setting of Defined Emission in three different Mode settings: Inline, Circular and 3D Grid.
Emitter Shape menu options, with the default, Rectangle, selected.