This is the index identifying one specific entry, from 1 to 16, in this port's traffic shaper table, since the ATM-IWU and CS cards support exactly 16 independent shapers per the description. Because only 16 shaper slots exist per card, this index on this specific port is really about which of a limited, fixed pool of shaping resources a given VC or traffic class is assigned to, so running out of available indices caps how many independently-shaped traffic streams this port's card can support at once. A card already using shaper indices 1 through 15 for existing circuits on this port would have only index 16 left to assign to any newly provisioned circuit needing its own independent shaping profile.