To PacketWise, if traffic goes through one router and doesn't necessarily go back through the same router, this would be considered asymmetric routing. Basically, if PacketWise does not see the SYN SYN/ACK pair, then it assumes this traffic is asymmetric.
Another situation in which PacketWise marks traffic as asymmetric is when the unit is reset in the middle of a flow. Once the unit comes back online, PacketWise will see all current flows as asymmetric. When the session is over and a new flow has been established with the SYN SYN/ACK pair, then PacketWise will see this traffic as symmetrical.
Also, if PacketWise sees a large jump in the packet sequence numbers, these packets will be marked as asymmetric.
What does this have to do with classification? The problem with asymmetric routing and auto-discovery is that if PacketWise does not see the beginning of the flow, it will not classify this traffic. It's possible you will come across flows that will not be auto-discovered for some time. These flows are usually long lived, and could last several hours and even days.
You can classify asymmetric traffic by manually creating the class -- if the class is port based (traffic that is discovered by the port number).
If it is relying on layer seven only, PacketWise will not be able to classify this traffic even if it already exists in the traffic tree.
For instructions on creating a class, see Create a Traffic Class in PacketGuide.