Image Numbers are specific to WIM imaging. You can store more than one image in a WIM file. Each image is identified by an image index, which is represented as the WIM Image Number.
The WIM image number can be used in incremental back up in which you can maintain different Images corresponding to different states of the device and can restore to any particular state by selecting the Image Number to restore to.
For a base Image, if the device had more than one partition, then each partition image has a unique image number. By using image numbers, you can restore a particular partition.
While restoring an Add On Image, you must specify the image number corresponding to the image to be restored.