It tracks both outgoing and incoming traffic, but yes, on the VR public side interface only.

One more question that I should have added, does it bill for outgoing traffic only or both?

The network bandwidth product in CPBM maps to bytes in and bytes out usage records obtained from CloudPlatform. CloudPlatform generates these records in two ways. As Vijaykumarn mentions, traffic on an isolated network is metered from the Virtual Router's public interface, both incoming and outgoing. For shared networks, traffic can be metered using inMon Traffic Sentinel, which is a third party software package.

The monetary rate set for the network bandwidth product applies to both incoming and outgoing traffic. So if your rate is $0.05/GB and you have 20 GB incoming and 30 GB outgoing in a month, charge is $0.05 * (20 + 30) = $2.50.