What characterizes a supplementary essential variable according to ASME code?

In the context of the ASME code, a supplementary essential variable specifically refers to criteria that are not fundamental to every welding procedure qualification but become relevant under certain conditions, such as when notch toughness testing is required. Notch toughness is a critical factor in ensuring that welds can withstand impact loads, especially in low-temperature applications or in situations where the material must be resistant to fracture.

When notch toughness testing is mandated, the supplementary essential variables can come into play to help ensure that the weld meets the necessary performance standards for toughness. Therefore, the recognition that these variables apply under specific circumstances underscores the importance of proper qualification processes in welding to guarantee structural integrity and performance.

The other options either misrepresent the nature of supplementary essential variables or are unrelated to the criteria set by the ASME code regarding welding procedures and qualifications.