Service Bus Premium Messaging provides resource isolation at the CPU and memory level so that each customer workload runs in isolation. This resource container is called a messaging unit.Each premium namespace is allocated at least one messaging unit. You can purchase 1, 2, 4, 8 or 16 messaging units for each Service Bus Premium namespace.

https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-premium-messaging