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Systems
Systems are where all the logic lives.
The way systems are designed there is an orchestration layer which wraps all systems and handles the communication between the pools and the execution/reaction/setup methods known as ISystemExecutor (Which can be read about on other pages).
You just express how you want to trigger your systems and let the SystemExecutor handle the heavy lifting and trigger the relevant method in the system when its time. This can easily be seen when you look at all the available system interfaces which all process individual entities not groups of them.
This only documents the SystemsRx available systems but EcsRx builds on top of this and provides many other system types and an ECS paradigm.

System Types

This is where it gets interesting, so we have multiple flavours of systems depending on how you want to trigger them, by default there is IManualSystem which acts as a simple setup/teardown style system. You can also mix them up so you could have a single system implement IManualSystem, IBasicSystem and IReactToEventSystem which would trigger all the required methods when system sets up/tears down, when an update happens and when an event comes in, but ultimately you can mix and match the interfaces however you want.

IManualSystem

This is a niche system for when you want to carry out some logic outside the scope of entities, or want to have more fine grained control over how you deal with the entities matched.
Rather than the SystemExecutor doing most of the work for you and managing the subscriptions it leaves it up to you to manage everything how you want once the system has been started.
The StartSystem method will be triggered when the system has been added to the executor, and the StopSystem will be triggered when the system is removed.

IBasicSystem

This is a basic system that is triggered every update (based on scheduler update frequency) and lets you do anything you want per update.

IReactToEventSystem

This allows you to react to any event of that type which is published over the IEventSystem.

System Loading Order

So by default (with the default implementation of ISystemExecutor) systems will load in the order you add them, however you can add a [Priority] attribute to indicate an explicit order for running.