-perpacket and flowDirection

I would like to know how the `flowDirection` feature is extracted if the `-perpacket` flag is set.

If one extracts single packets (a conversion of a pcap into a csv), I thought that we do not care much about the flow key and hence, there is no way to get the direction, but it seems that go-flows creates flow tables but then writes an entry each time a packet is fed. Is this the case?