Answer
Hierarchical QoS allows you to control QoS at multiple levels: the physical level, the logical level, and fine-grained control at the command-line level. It is useful for managing bandwidth congestion and link sharing in multi-service networks. For MX Series devices with AE interfaces, hierarchical QoS per VLAN is available over a link aggregation group (LAG) in Junos OS Release 9.4 and later. This support is only one-to-one active/backup. However, it can be supported across DPCs.
For more information, see MX Series Interface Module Reference