This article describes the difference between the different kinds of bandwidth provisioning for interface queues that are used in schedulers.

The exact (transmit-rate) command on queues is a shaping action; for example when a queue reaches its transmit-rate with exact turned on, all subsequent packets are just buffered.

The rate-limit command queues is a policing action; for example when a queue reaches its transmit rate, subsequent packets will be dropped. 

• JUNOS 9.5 Config-guide-cos transmit-rate