This is the index (1 to 4) identifying which of the router's satellite receive interfaces a given row of Rx statistics applies to. Since this CDD-564 model supports up to four satellite Rx paths, this index is what lets SNMP tools correlate a specific frame or header-byte count with the physical demodulator it came from, rather than aggregating all four into one ambiguous number. A management station polling a four-demodulator unit would walk this index 1 through 4 to build a per-interface table, allowing it to spot that demodulator 3 specifically is showing elevated error counts while 1, 2, and 4 look healthy.