Describes how this group fits into the hierarchy. A
positive value is interpreted as an mtaGroupIndex
value for some other group whose variables include
those of this group (and usually others). A negative
value is interpreted as a group collection code: Groups
with common negative hierarchy values comprise one
particular breakdown of MTA activity as a whole. A
zero value means that this MIB implementation doesn't
implement hierarchy indicators and thus the overall
group hierarchy cannot be determined.