Applying Alternative Promotion in Apply Campaign Transaction

If campaigns that are applied to the sales slips and invoices are saved with promotional material class,
the first material of the class is displayed as the promotion line. However, if you need to select from the other materials of this class, use "Select Alternative Promotion" option located in F9-Operations menu of material class promotion lines which are applied campaign. This option is active only when the material type of the promotion line in the campaign is material class. 
All child materials of the material class that is defined in promotion campaign are listed with physical and actual inventory quantities in Select Alternative Promotion window. The quantity can be distributed among child materials providing that it is equal to the total line quantity. 

Example; 
Let's assume that a campaign offering 3 pieces of material B upon selling 10 pieces of material A is applied to the invoice, and child material B1 is given as promotion:

Child materials of the material class are listed by clicking Select Alternative Promotion option in promotion line.

Promotion quantity can be distributed among the other child materials.

The invoice will be as below after changing the quantity and clicking OK.