Processing logic for return traffic in stateful inspection

[KenjiKang]

Hi teams

I am using a FortiGate firewall with stateful inspection enabled, and we'd like to understand the packet processing logic for return traffic.
Could you please share any information you have on this?

Specifically, I am looking for an explanation of return traffic similar to the one provided in the following URL: https://docs.fortinet.com/document/fortigate/6.0.0/parallel-path-processing-life-of-a-packet/478386/...

I am currently considering whether to address return traffic by adding a policy route or by adding a less-preferred static route. Our main question is whether the routing table is consulted at all for stateful return traffic.

Thank you 
Kenji