Ok, that is a basic question.
I have two uplinks (ether2, ether3) and a pptp-link (link-nue).
I have a queue tree doing prioritization under link-nue. Subitems. Works FINE.
I just added 2 queue trees under ether2 and ether3 out queueus.
[Administrator@link-szn] /queue tree> print
Flags: X - disabled, I - invalid
0 name=“link-nue-out-tmd” parent=link-nue-out packet-mark=timed limit-at=64k
queue=distributed-default priority=2 max-limit=512k burst-limit=0
burst-threshold=0 burst-time=0s
1 name=“link-nue-out-int” parent=link-nue-out packet-mark=interactive
limit-at=64k queue=distributed-default priority=3 max-limit=512k
burst-limit=0 burst-threshold=0 burst-time=0s
2 name=“link-nue-out-unm” parent=link-nue-out packet-mark=no-mark
limit-at=16k queue=distributed-default priority=8 max-limit=512k
burst-limit=0 burst-threshold=0 burst-time=0s
3 name=“link-nue-out-slo” parent=link-nue-out packet-mark=slomo limit-at=64k
queue=distributed-default priority=7 max-limit=512k burst-limit=0
burst-threshold=0 burst-time=0s
4 name=“link-nue-out-req” parent=link-nue-out packet-mark=requested
limit-at=64k queue=distributed-default priority=5 max-limit=512k
burst-limit=0 burst-threshold=0 burst-time=0s
5 name=“link-nue-out” parent=link-nue limit-at=512k priority=1
max-limit=512k burst-limit=0 burst-threshold=0 burst-time=0s
6 name=“ether2-out” parent=ether2-uplink limit-at=512k queue=default
priority=1 max-limit=512k burst-limit=0 burst-threshold=0 burst-time=0s
7 name=“link-nue-out-inf” parent=link-nue-out packet-mark=infra limit-at=32k
queue=ethernet-default priority=1 max-limit=128k burst-limit=0
burst-threshold=0 burst-time=0s
8 name=“ether3-out” parent=ether3-uplink limit-at=512k queue=default
priority=1 max-limit=512k burst-limit=0 burst-threshold=0 burst-time=0s
The problem: While the link-nue queue works, the other 2 wueues seem to see no traffic at all (statistics stay at 0).
How would I handle this?
I Want to have a queue for PPTP, and another one for every ethernet outlink (that will prioritize GRE / the pptp link).
I can put the main queue into global out, but that will not be nice with the limits (because I have multiple outlinks so the banfwidth limits must apply per connection / interface).
Under another concept - how do I mangle the output packets of a PPTP connection with a packet mark so athat they can get proper priority on the next queue tree? 
Would it be post-routing, which serves as input for pre-routing then? (as the PPTP packet then needs to be routed)?