The HPLMIXED Procedure
CLASS variables ;
The CLASS statement names the classification variables to be used as explanatory variables in the analysis. These variables enter the analysis not through their values, but through levels to which the unique values are mapped. See the section Levelization of Classification Variables of Chapter 3: Shared Statistical Concepts, for details about these mappings.
If a CLASS statement is specified, it must precede the MODEL statement in high-performance analytical procedures that support a MODEL statement.
Levels of classification variables are ordered by their external formatted values, except for numeric variables with no explicit format, which are ordered by their unformatted (internal) values.