This index uniquely identifies a logical entity (such as one modem channel) within the chassis using the formula E = 1000*S + I, where S is the slot number and I is that entity's position on the card, so the index itself directly encodes both which slot and which sub-unit on that card a row describes. An admin benefits from this because they can derive the physical slot from the index alone without a join against the slot table -- dividing by 1000 immediately tells them where to look. For example, an entity index of 5003 (as the MIB's own example states) decodes as slot 5, entity 3 -- the third modem on a quad-modem card in that bay -- letting an admin pinpoint a failing channel directly from an alarm's index value.