Systems are nothing but a specialized combination of a few Units, a System creates a few predefined relationships among its member Units to make them part of a whole.
While the Units are storage mediums, Systems are task helpers.
A System tries to accomplish a specific type of task (that has multiple aspects to it, and hence requiring multiple Units to work in cohesion) in a reactive and shared fashion, such as data fetching using XHR API, or fetch API. Both of these APIs or any other asynchronous API more or less involve a query, a successful response, an error, and pending-status of the task.
The AsyncSystem is the System that is built to help with these kinds of asynchronous APIs.