As explained here, the "Data Partition" option creates a buffer that is untouched when the OS is installed. Like any partition, it segregates memory so that if a OS is reinstalled to the SD card, it doesn't wipe out all the preexisting memory.
Data Partition - Add a 512mb data partition for data that will be kept safe during a re-instalation of an operating system.
A source here goes into further detail on what it does.
It sounds to me like selecting that option will allow you to create a 'recovery disk' of sorts. It might have come as a result of discussion on this issue, where people requested a partition that persists despite wiping another partition on the Pi.
I hope this helps!