The document describes how to boot a BeagleBone Black from an SD card containing two different root file systems on separate partitions and run programs from each one. It involves cross-compiling two simple C programs that print identifying messages and copying them to their respective partitions. By changing the mmcroot setting in uEnv.txt to point to either the second or third partition, the board can be booted to run one program or the other from its dedicated root file system on subsequent reboots.