This statistic measures network device activity in bytes/sec. Network devices are the physical network ports, and are shown in the Device column of Network. The measured bytes by this stastistic includes all network payload headers (Ethernet, IP, TCP, NFS/SMB/etc.)
Network bytes can be used a rough measure of appliance load. It should also be checked whenever performance issues are investigated, especially for 1 Gbit/sec interfaces, in case the bottleneck is the network device. The maximum practical throughput for network devices in each direction (in or out) based on speed:
1 Gbit/sec Ethernet: ~120 Mbytes/sec device bytes
10 Gbit/sec Ethernet: ~1.16 Gbytes/sec device bytes
If a network device shows a higher rate than these, use the direction breakdown to see the inbound and outbound components.
Also see Network: Interface bytes for network throughput at the interface level, instead of the device level.