System service instances are the result of a running system service. For example, system services that are stopped for any reason do not have system service instances.
A system service instance only runs one activity. You cannot control the activity directly, only the system service instance that contains it. For example, you can migrate an instance (and therefore the activity it contains), but you cannot migrate the activity directly.
You can have multiple system service instances running under a single system service; this means multiple activities can run simultaneously.
By default, system services have a configured number of system service instances to run on each slot (1 instance per slot).
System service instances require a host to run an activity. A system service instance runs using the resource group (for example, the ManagementHosts group or the ComputeHosts group) that is defined in the system services' definition files. Find the definition files here: