Azure Service Bus recently upgraded the billing components. Because of this change, if you have a Service Bus Standard namespace, you may see line items for the resource '/subscriptions//resourceGroups/$system/providers/Microsoft.ServiceBus/namespaces/$system' under resource group '$system'.
These charges represent the base charge per Azure subscription that has provisioned a Service Bus Standard namespace.
It's important to note that these charges aren't new, that is, they existed in the previous billing model too. The only change is that they're now listed under '$system'. It's done because of constraints in the new billing system that groups subscription level charges, not tied to a specific resource, under the '$system' resource ID.
Quotas
For a list of Service Bus limits and quotas, see the Service Bus quotas overview.