bitsPortTypeIndex is a read-only integer that identifies whether a given row describes the BITS-IN (value 1) or BITS-OUT (value 2) direction of a Building Integrated Timing Supply port, one of the indices used to uniquely address a BITS port row. An administrator relies on this distinction because BITS-IN receives an external timing reference into the chassis while BITS-OUT distributes the chassis's selected clock outward, and many other BITS objects (like squelch and line build-out) apply only to the OUT direction. For example, when reading the BITS port table an SNMP client checks this index to know whether it is looking at the reference-input row or the timing-output row for a given card.