pDirectionalForce [pDF]

This tool applies a uni-directional force that pulls the affected particles in a specified direction. Its primary controls affect the strength of the force, and the angle of the forces pull along the X, Y and Z axis.

As the most common use of this tool is to simulate gravity, the default direction of the pull is down along the Y Axis (-90 degrees) and the default behavior is to ignore regions and affect all particles.