Those are typical J Factor(base metals) X Factor (filler metals) elements that determine a materials susceptibility to reheat cracking or temper embrittlement. typical of CrMo type, and other low alloy type materials.
If you google X Factor, J Factor, Watanabe, or Bruscato (the developers of the formulas) you will find much information.
These Factors are formulas that result in a number that can be limited by specification. Often in the neighborhood of 15.
Also, the WRC Welding Research council has a really informative bulletin on Temper Embrittlement, discussing what metllurgically takes place with these elements and the suceptible temperature ranges.