Root shielding gas
Shielding gas specifically routed to the underside of a joint to prevent root oxidation and burn-through.
When root welding, it's important to purge the weld from the back side with shielding gas. This displaces oxygen that would otherwise oxidize the hot root. Without purging, you'll get discoloration, porosity, and with stainless steels the weld loses its corrosion resistance because the chromium burns off. You have to let the gas flow through the pipe or vessel long enough before welding until the air is out. How long that takes depends on the internal volume – when in doubt, purge too long rather than too short. During welding, let the gas flow and continue purging afterwards until the root has cooled. This keeps the weld clean and tight.