SATA data + power cable
This is an optional cable for clients that want to connect a full size HDD/SDD or even CD-ROM to the APU board.
It's not required if you are getting the mSata drive. This is only for the regular, full sized, additional devices. You may want to get it if you are planning on adding additional storage to your system.
There's enough space in the standard enclosure for one 2.5 inch drive.