This is simply the position number assigned to a cooling fan, counted from the front and/or left side of the chassis, used as an index so other fan-related objects like atiFanOperational in the same table row can be matched to a physical fan. It matters when a technician needs to walk up to the actual hardware and pull the correct failed fan module without guessing, since the SNMP table alone doesn't show a picture of the chassis. For example, a value of 2 combined with a failed atiFanOperational reading tells the technician to look at the second fan slot from the front-left when replacing it.