HP StorageWorks MSA 2/8 - SAN Switch Reference Manual page 172

Entity MIB Objects
Description
172
An indication of the relative position of this child component
among all its sibling components. Sibling components are
defined as entPhysicalEntries that share the same instance
values of each of the entPhysicalContainedIn and
entPhysicalClass objects.
An NMS can use this object to identify the relative ordering
for all sibling components of a particular parent (identified by
the entPhysicalContainedIn instance in each sibling entry).
This value should match any external labeling of the physical
component, if possible. For example, for a container (such as
a card slot) labeled as "slot #3," entPhysicalParentRelPos
should have the value "3." Note that the entPhysicalEntry for
the module plugged into slot 3 should have an
entPhysicalParentRelPos value of "1."
If the physical position of this component does not match any
external numbering or clearly visible ordering, then user
documentation or other external reference material should be
used to determine the parent-relative position. If this is not
possible, then the the agent should assign a consistent (but
possibly arbitrary) ordering to a given set of sibling
components, perhaps based on internal representation of the
components.
If the agent cannot determine the parent-relative position for
some reason, or if the associated value of
entPhysicalContainedIn is 0, then the value "-1" is returned.
Otherwise a non-negative integer is returned, indicating the
parent-relative position of this physical entity.
Parent-relative ordering normally starts from 1 and continues
to N, where N represents the highest positioned child entity.
However, if the physical entities (for example, slots) are
labeled from a starting position of zero, then the first sibling
should be associated with an entPhysicalParentRelPos value
of "0." Note that this ordering may be sparse or dense,
depending on agent implementation.
MIB Version 3.1.x/4.1.x Reference Guide