The "Index" column shows you the index of each device within its device type group.

Normally this will be "1" for each device. However, if you have multiple devices of a certain type, each such device will receive a unique index.

For example, if you have two CD-ROM drives, they will receive the indices "1" and "2", respectively.

Note that if you have IBM's Netscape Plug-in Pack with MPEG support installed, the MPEG support is installed as a second digital video device as well for some reason.