SAS code for hot decker imputation

I am just learning about using the hot decker imputation method to impute values in cases where values are missing in survey. I want to impute values for demographic variables such as age, health insurance, education, income, and marital status. All of the variables are categorical except for age. I found a macro for unweighted imputation method.

Can someone explain to me what is considered a class variable and what is a sorting variable in my situation? Are categorical variables class values and continuos variables sorting variables?