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The Particle System nodes are automatically created by Phoenix in order to expose several different particle systems (groups) available in only a single node. Such is the case with Phoenix Simulator nodes which load data from Phoenix cache files and can expose several particle systems at once - Foam, Splash, Liquid, WetMap, Mist, or Drag particles. Additionally, Thinking Particles nodes can also contain many particle systems. The nodes are named "Particles [...] of [...]" where the first slot is the name of the particle system that is represented, such as Foam, Splash, etc., and the second slot is the name of the node that provides the many particle systems.
The Particle System nodes are required in order for other nodes such as the Phoenix Particle Shader, the Phoenix Source, or Thinkbox Frost, to be able to select only a specific particle system in case more than 1 are available.
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The Krakatoa PRT Loader and Phoenix PRT Reader, on the other hand, only provide one single particle system so they can be directly picked by Sources or Particle Shaders. Also, the Phoenix Simulator itself provides a mixdown of all available particle systems if picked directly by another node that wants to obtain its particles. This is why when exporting Alembic from 3ds Max's built-in exporter, you can get separate particle systems from each Particle System node, or you can switch the Simulator's Export as option to Particles and you will get a mixed group of all the particles of the Simulator exported.
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