The Odroid C5 eMMC cards are exclusive to this board, and areavailable in 16GB, 32GB, 64GB, 128GB, and 256GB  sizes and they come preinstalled with an OS installer.

This means that when you switch the Odroid C5 unit on with the eMMC card attached, you are presented with a "one-time-only" menu that allows you to choose which Operating system you would like to install to the card. 

Once you have chosen your OS, you leave the board switched on, and it will install the chosen operating system to the eMMC card for you.

ioZone Test Results

If a reflash of the card is required, you would need to do so using one of the below writers:-

USB3 to eMMC Reader
Micro SD eMMC Card Reader

Further Information

Wiki : https://wiki.odroid.com/accessory/emmc/emmc

Schematics : http://dn.odroid.com/eMMCModule/