To enable data caps, you need to have a router or firewall that supports NetFlow version 5 in your network. You also need to configure it to send NetFlow data to NxFilter. And then run NxFilter's built-in NetFlow collector on 'System > Setup > NetFlow'. After that, you can set data cap on a policy.
There are several rules for NxFilter to import NetFlow data. Firstly, one of the source or destination IP address of a NetFlow data should be associated to an IP address of a logged-in user on NxFilter. Secondly, NxFilter ignores internal traffic. And one of the source or destination IP address needs to be a public IP address. This is because you are only interested in inbound or outbound traffic from/to the internet. And lastly, NxFilter keeps only TCP/UDP data.