Hi Terence,

When using multi-machine rendering, only deterministic particles should be used.

In the case of the Niagara Particle system, you need to turn on Determinism in each node in the particle system:

You can read more about the Determinism setting here: https://dev.epicgames.com/documentation/en-us/unreal-engine/emitter-settings-reference-for-niagara-effects-in-unreal-engine 

Unreal is a game engine, these particle systems were designed to appear in games and be randomized so you wouldn't see repeating patterns.
If you don't have Determinism turned on, then the particle system will be different each time you run it, even on the same computer.

In the multi-machine rendering, individual Unreal instances are running on each computer, and these instances don't have direct communication with each other. The instances are started from the same folder, but they have their own internal state.
Each Unreal instance has its own camera or render texture, however, these are synced to a degree by Aximmetry Composer...

If you don't turn on Determinism, each Unreal instance will generate random values for the particle system based on your computer's internal temperature (or by using similar tricks). If you turn on Determinism, then Unreal will generate the random values by using an algorithm. In this case, the random values will be the same each time you restart the algorithm (when you start Unreal). This way, Determinism ensures that you get the same-looking particle system each time you run an Unreal Scene. For example, this also ensures that you can render the same footage for a video each time you start Unreal. It also ensures that you see the same-looking particle system in Aximmetry multi-machine as long as on the different computers the running Unreal scene instances are not different and the particle systems have similar internal states.