Answer
A forwarding class is a label, used entirely within a network node, that is used to identify all traffic that requires a single behavior when leaving that node. You map incoming traffic to a forwarding class, define the class-of-service (CoS) properties for the forwarding class through a scheduler, and assign the forwarding class to an output queue. For most CoS applications, you assign one forwarding class to one output queue. But for some CoS applications, you can map multiple forwarding classes to a single output queue.
Juniper routing devices provide four default forwarding classes: best effort, expedited forwarding, assured forwarding, and network control. On most Juniper routing devices, you can configure up to 16 custom forwarding classes mapped to up to 8 queues.
For more information on forwarding classes, please see https://www.juniper.net/techpubs/en_US/junos15.1/topics/concept/forwarding-class-overview-cos-config-guide.html.