There are three different places you can set POP Collisions. You can use the Collision Behavior tab on the 
POP Solver, the 
POP Collision Behavior node, or the POP Collision Detect node. Depending on what you want to accomplish, you can set up collisions using any of the following:
Use the Collision Behavior tab on the POP Solver for doing simple RBD collisions. Particles will bounce by default, but you can also set them to die, stop, stick, or slide.
Use POP Collision Behavior when you want the particles to do more than one action. For example, have some particles bounce, some stick, and some slide.
Use POP Collision Detect when you don’t necessarily want to change the behavior of the particles, but gather information such as when particles hit, where they hit, what their velocity was when they hit, etc.
You could also use this node to generate attributes that can be applied to specific particles. For example, when particles hit a surface, make them turn red.
