Behavior of Quote Line field SBQQ__AdditionalDiscount__c in Salesforce CPQ

In a fresh install, when added to the quote line's Line Editor field set, the field SBQQ__AdditionalDiscount__c on the Quote Line appears as two fields when selected. The field on the left allows for the entry of a number (to the precision defined in the "Unit Price Scale" field in the General Settings) and the field on the right is a picklist that allows the user to select a Percent discount or an Amount discount.

If a % discount is selected, the Quote Line field SBQQ__Discount__c populates with the user-defined value.
If an Amount discount is selected, the Quote Line field SBQQ__AdditionalDiscountAmount__c populates with the user-defined value. 

CONSIDERATIONS

• The above fields are populated from SBQQ__AdditionalDiscount__c before On Initialization price rules execute. 
• Only one of the above fields may be populated on a given Quote Line. If both are populated, a managed Validation Rule fires stating:
  • "You may not specify additional discount as both percentage and absolute amount."
• Price Rule Consideration: since the SBQQ__AdditionalDiscount__c has the field definition "(SBQQ__RegularPrice__c - SBQQ__CustomerPrice__c) * SBQQ__EffectiveQuantity__c" it does not behave in the same manner as SBQQ__Discount__c or SBQQ__AdditionalDiscountAmount__c when referenced for Price Conditions, Price Actions, etc. 

Note: your admin can include or exclude picklist values that directly impact how users are allowed to discount Quote Lines using the SBQQ__AdditionalDiscount__c in the Quote Line Editor. "Unit" and "Total" values may be added as additional picklist values within the Line Editor for the SBQQ__AdditionalDiscount__c field. See here for more information.