both are use, HGI is going depend what type of coke are you produce is a measure of the relative hardness of the coke in terms of resistance to grinding. An HGI below 35 usually means a very hard coke requiring considerable grinding before it can be properly sized to meet burner nozzle specifications for the pulverized feed to the furnace where the coke will be burned. An HGI above 65 is usually recognized as a soft coke that will grind very easily.
the VCM in Pet coke from a coker contains a small amount (<10%) of light hydrocarbons trapped in the pores of the coke. The amount of such volatility is generally related to the how hard the coker furnace is driven: hotter furnace outlet temperatures result in more hydrocarbons driven off and therefore lower volatility. normally is between 8-12%
other spec is a Moisture content in Pet coke as cut from a coker contains roughly from 7-10% moisture which the coke picked up from the steaming and quench operations. It is measured by heating a 100g sample of ground coke, until the moisture is gone, and then re weighing.