From Technologic Systems Manuals

The TS-8551 has a block of jumpers located near the rear of the PCB. These jumpers control a number of aspects of the platform's behavior. The jumpers are labeled on the silkscreen rather than numbered:

Label Description
SD Boot When jumper is set, boot kernel and Os from the SD card. Otherwise boot kernel and OS from eMMC.
No Charge When jumper is set, disable charging of the TS-SILO Supercapacitors. Beneficial for early development and testing.
U-Boot When jumper is set, U-Boot attempts to boot from USB before dropping to a U-Boot shell. Otherwise boot straight to Debian.
TS-4400 Required to support booting a TS-4400 on the TS-8551. This should be left open when the TS-8551 is used with a SoM other than the TS-4400.
SBC 5V On Used for internal engineering/development. This should be left open.