By default, Commerce includes messages used by the stacking rules feature. To enable messages related to this feature, set the following property to true:

PricingTools.sendStackingRuleMessages

By default, Commerce includes a StackingRuleMessages message group that is used to send messages for that feature.

The default slot for stacking rule messages is /atg/commerce/pricing/CleanBeforePricingSlot. Messages are not removed from this slot as they are retrieved. However, by default, the slot is configured in the MultiSlotMessageTools.slotsToClean property, so messages are cleared automatically at the start of each pricing operation.