Many particle nodes display "guides" in the viewer when you turn on the node’s template flag. The guide display varies depending on the operator. For example:
Operators that apply forces to the particles display lines showing the direction and power of the force. This lets you visualize the effect of forces and is useful for debugging aberrant behaviors such as very large or nonexistent forces.
For example, this image shows the guides (in blue) of a Wind POP, showing the force (in blue) it is applying to each particle to blow it to the right.
The guide for the Instance POP previews the instanced geometry in the viewer.
The guide for the Limit POP displays the limit boundary in the viewer.
The guide for operators that reference geometry, such as the Collision POP, shows the referenced geometry.