Netflow and local net accounting

Hi ! I want to count trafic between users in local net (and of course internet trafic). In the netflow statistic I can see interface number on which was sent packet. Can you explain me what logical relation between this number and interface number in the Mikrotik ? Another words I must to have hard link - interface number in netflow and interface number in Mikrotik (/interface print). This is for right accounting trafic in the Local net only (double trafic).
Thank’s.