The STABO harness was a device worn by soldiers so they could be extracted by helicopter or the Fulton surface-to-air recovery system. The harness could be worn over most forms of Sneaking Suits and Battle Dresses.
Big Boss used the STABO harness during the Virtuous Mission, Operation Snake Eater, and the Peace Walker Incident. During the Peace Walker Incident, Big Boss and the Militaires Sans Frontières soldiers used the harness with the Fulton system for extraction at the end of missions.
At some point prior to the events of the Ground Zeroes incident, Big Boss and Venom Snake transitioned to the SPI/E harness system, a more modern equivalent adapted from the STABO harness. Venom Snake continued to use the SPI/E during the Phantom Pain Incident. However, at least some Mother Base personnel continued to utilize the older STABO harness system.
Behind the scenes
The STABO harness appears to be standard issue for many factions in the Metal Gear series.