The HS stands for the following:
H is for "high carbon", but actually is for a controlled carbon content, normally 0.04 to 0.08% carbon. For example, a 316 stainless steel filler could have the following carbon levels:
316 would be 0.08% maximum, with nominal something less than 0.08%, but could be as low as 0.02%.
316L would be 0.04% maximum, with nominal as low as 0.01% to 0.02%.
316H would be 0.04% to 0.08%, with nominal at 0.05% to 0.06%.
The reason for the "H" grade is to control the carbon content for improved performance at elevated temperatures.
S is for a "higher sulfur" content and is elevated to improve bead shape and penetration effects. The "GTAW Flux" has enhanced penetration characteristics, with one of the elements being sulfur.
Check out the short article attached.