When using the gravity force, set this value between 0 and 1 to control the focal point of the gravity effect.
Setting a value of 0 causes particles to be attracted to the center of the volume, whereas setting a value of 1 will result in particles being attracted to the outer edge of the shape. Setting a value in between 0 and 1 will move the focal point between the inner and outer extents of the volume. Additional resources: ParticleSystemForceField.