PCC "chain=policy_routing" versus "chain=prerouting"

Some PCC examples use “chain=policy_routing” and other use “chain=prerouting” for the “action=mark-connection” with
“per-connection-classifier” mangles.

For instance http://mum.mikrotik.com/presentations/ZA13/bisschoff.pdf versus http://mum.mikrotik.com/presentations/US12/steve.pdf

What’s the differences between using “chain=policy_routing” and “chain=prerouting”? Why would you choose either one?

–jeroen