Commissions are typically processed using the normal commission rates, defined in the Commission Rate Specifications. If you have certain promotions and different commission rates for those, you can define those as well. If you are using sales campaigns to define your promotions, you can enter commission rates for the specific sales campaigns. Only, if those campaigns are defined on the sales document, the system will use the specific promotional rates. Otherwise, the normal rates are used.
If you do not use sales campaigns for your promotions, you can still define different commission rates for the duration that your promotion is running. However, since those have to be defined in the normal Commission Rate Specifications, those rates are applied to every sale during the promotional period.