This feature allows you to create a question with the selected sub-populations as answer categories.
To access this feature, in the edit menu select create a variable..., then select by sub-population. Select the sub-population(s) in the right-hand list, then click the following button to move them into the left-hand list:

.You can insert a subtotal by clicking
.
To delete a sub-population from the left-hand list, select it and click
.
To rename a sub-population, select it and press F2.