How zabbix calculate bits send and receive (net.if.in and net.if.out)?

[Miguel Pereira]

Does anyone know how zabbix calculates the number of bits in and out (traffic statistics on a network interface)?
Is it averaged per second or just the sum of all bits received and sent in the time interval?

Also, does it monitor communications inside the machine? Or just into and out of the local machine?

Thanks!!!