Android M TWRP Backup, what is "System Image?"

First time using the backup feature in TWRP and I've been searching through guides for info. From what I've found, all I need for a complete backup is system, data and boot. However, I noticed there is also an partition called "System Image" that I can't find mentioned in any backup guides or info.

Anyone know what that is?

Options:

System (1952MB)
System Image (2048MB)
Data (5083MB)
Cache (0MB)
Boot (16MB)
Recovery (16MB)
EFS (3MB)

Thanks!