If an administrator wants to apply QoS to traffic based on source, they must specify the post-NAT source address in a QoS policy rule. This is because QoS is enforced on traffic as it egresses the firewall, and the firewall applies NAT rules before QoS rules. Therefore, the firewall will match the QoS policy rule based on the translated source address, not the original source address. If the administrator uses the pre-NAT source address in the QoS policy rule, the firewall will not be able to identify the traffic correctly and apply the desired QoS treatment. Reference:
QoS Policy
Configure QoS