There is a difference in a back weld and a backing weld.
A back weld is performed after back gouging as indicated by Dividstecker.
A backing weld serves the same function as a backing bar.
The back weld is the "last" weld deposited to "fill" the groove that results when the second side is back gouged to remove any unfused material or weld in the root that might be compromised.
The backing weld is the first weld bead deposited that supports the root bead while it is in the liquid state and cools and solidifies. A typical application could be submerged arc welding. When the joint is assembled, there may be some root opening (gap) between the two members. A backing weld is deposited to "seal" the joint so the granular flux doesn't simply flow through the root opening. So, the backing weld seals the joint to keep the flux in place. Once the root bead is deposited, the backing weld has served it purpose. If it was deposited on the root side of the joint, it can be removed by back gouging and back welded to ensure the weld is sound.
Once the procedure is qualified, the production welds can be made with or without the backing weld or with or without backing because the backing is a nonessential variable.
Best regards - Al