Back up electrodes are used for projection welding electrodes. They are normally used on the lower side and are flat faced. There is no need for a defined weld face since the projection concentrates the weld current and heat into a desired spot for nugget creation.
Back up electrodes are also used to help in heat balancing and cosmetic welds. This is where you want to reduce the heat affects on the one side of the weld joint. By using the large flat face the current is spread out over a large surface and effectively cools that face.
Projection welding schedules and electrode recommendation can be found in the Resistance Welding Manual and AWS Standard C1.1 - Recommended Practices for Resistance Welding both published by RWMA/AWS.
Lower electrode is a back up electrode to support the part and dissipate heat during this projection welding operation.
Reference: RWMA - Resistance Welding Manual 4th Edition