This variable uniquely identifies the group
containing the port to which a power Ethernet PSE is
connected. Group means box in the stack, module in a
rack and the value 1 MUST be used for non-modular devices.
Furthermore, the same value MUST be used in this variable,
pethMainPseGroupIndex, and pethNotificationControlGroupIndex
to refer to a given box in a stack or module in the rack.