JMP stepwise regression uses hierarchical coding for categorical variables (nominal and ordinal), and the variable name {a&b&c&d&e-f&g} indicates that the variable levels are split into two groups: (a,b,c,d,e) and (f,g).
For details, see the "Models with Nominal or Ordinal Terms" section in the "Stepwise Regression Models" chapter of the "Basic Regression Models" online help.
FAQ # 475
