In a Particle System, the World Size determine the boundary of the simulation, of where the particles may go. The size is determined by the XYZ axis. It's quite standard to most simulation.
As the Particle System 3D receive a 2D texture as detection, the Interaction Surface applies the 2D detection texture to the 3D world.
The Interaction Surface may be set On All Volume, meaning the Interaction Surface is stretched to fit the World Size.
If not, the Surface Size and Position determine can be set and only the particle reacts only when their X and Y position are inside the Interaction Surface. Note that all positions are in absolute 3D value.
In both cases, the particle reacts to the Interaction Surface regardless the Z position.
The Interaction Surface is where the detection texture is applied in the 3D world.
The Debug Output Pin shows the World Size and the Interaction Surface.