I’m looking for a definite answer for what happens when in a mangle subchain called from Prerouting:
If a rule in ChainA matches and it has Passthrough = no, will it return to the main Prerouting chain or will break the Magle-Prerouting completely?
from documentation
https://help.mikrotik.com/docs/display/ROS/Mangle
passthrough (yes|no; Default: yes) whether to let the packet to pass further (like action passthrough) into the firewall or not (property only valid some actions)
The way I understand passthrough=no will stop evaluating further filters (for that package) for the mangle process.
Here it is explained a bit better:
http://forum.mikrotik.com/t/confused-with-passthrough-yes-no-in-mangle/130593/2