When you enter multiple By variables, Minitab enables the Show all combinations checkbox. Select this option to create a separate heatmap for each combination of groups created by the By variables. If you do not select this option, Minitab creates a plot for each group of each By variable.
For example, the first By variable has two groups, Male and Female, and the second By variable has two groups, Employed and Unemployed. If you select Show all combinations, Minitab creates four separate plots for the combinations of Male/Employed, Male/Unemployed, Female/Employed, and Female/Unemployed. If you do not select Show all combinations, Minitab creates four separate plots for Male, Female, Employed, and Unemployed.