The flow of traffic from a host A to host B consists of packet exchange in two directions (A->B and B->A<reply>). These are considered two different unidirectional flows.
The Palo Alto Networks firewalls, with the exception of the PA-4000 Series, support unidirectional NetFlow. The firewall expects to see traffic flow from A to B and from B to A. However, when NAT is involved, the flow appears from A to B and B to the natted IP of A.
Bidirectional NetFlow:
The flow of traffic from a host A to host B is considered just one flow, from A->B. This includes the sum of packets from A->B and B->A.
Bidirectional NetFlow is not supported on Palo Alto Networks devices.