Siemens SIMATIC NET Manual page 17

Index for Every Component
To allow communication between the components in the PC station and to receive
project engineering data, each component is assigned a unique identification
number. The identification number for modules, applications, and other
components in a PC station is the index. Analogous to the slot of a module in an
S7-400 controller, the index corresponds to a virtual slot in a PC station.
Note
Be careful not to confuse this "index" with a hardware slot, for example on the PCI
bus of the computer. The slot on the PCI bus is not relevant for commissioning
and is not used at any point.
Commissioning PC Stations - Manual and Quick Start
Release 5/2005
C79000-G8976-C156-07
1 Welcome to Advanced PC Configuration
17