Answer
If you do nothing to configure or assign classifiers on Juniper routing devices, Junos OS automatically assigns an implicit default IP precedence classifier to all logical interfaces that maps IPv4 precedence code points to best-effort and network-control forwarding classes.
By default, the best effort forwarding class (queue 0) receives 95 percent of the total port bandwidth and buffer space for the output link, and the network control forwarding class (queue 3) receives 5 percent. The default drop profile causes the buffer to fill completely and then discard all packets until it has space.
The expedited-forwarding (queue 1) and assured-forwarding (queue 2) classes have no reserved bandwidth or buffer space because, by default, no schedulers are assigned to those forwarding classes. However, you can manually configure resources for the expedited-forwarding and assured-forwarding classes.