Edge distance is a very important design criteria in resistance welding. Any weld on an edge can expel metal and or result in a partial weld nugget. This type of weld is considered deficient in quality programs. Edge welds should be avoided in all design and set up situations.
Definitions of edge welds can be found in AWS D8.1, section 5.1.1. Per D8.1 anytime the impression left by the electrodes, which is not fully contained within all original edges of the sheet metal is a discrepant weld.
References: RWMA - Resistance Welding Manual 4th Edition
AWS Standard D8.1M- Specification for Automotive Weld Quslity Resistance Spot Welding of Steel