In NFV solutions, the data traffic has two patterns, north south and east west.
North to south traffic comes from the hosting server into a physical NIC and is sent to a VNF.
The traffic is then sent to the VNF back out of the physical wire through to the physical NIC.
East west traffic comes into the hosting server through a physical NIC and is sent to a VNF. From there it may be sent to another VNF and possibly service chained to many more VNFs then sent back out of a physical NIC to physical wire.
There can combinations of both where a VNF uses a north to south pattern for user data and a east west traffic pattern to send traffic to a VNF that is collecting statistics or is being used for logs or storage.