@Abdul_Razaq they're related, but do different things in their own context. The PBF option is when you could have asymmetric routes, whereas in ECMP it overrides the inherent load balancing that ECMP provides. Both of the following are pulled from the inline help on the firewall from their respective sections:
Symmetric return in ECMP
Select Symmetric Return to cause return packets to egress out the same interface on which the associated ingress packets arrived. That is, the firewall will use the ingress interface on which to send return packets, rather than use the ECMP interface, so the Symmetric Return setting overrides load balancing. This behavior occurs only for traffic flows from the server to the client.
Symmetric return in PBF