What are Twin Fields in CPQ and how are they used?

Twin Fields is a CPQ mechanism for mapping field values from a Quote Line (SBQQ__QuoteLine__c) to the corresponding Subscription record (SBQQ__Subscription__c) at contract creation, and optionally from Subscriptions to Renewal Quote Lines at renewal generation. Use case: If you have a custom field SBQQ__QuoteLine__c.Custom_Discount__c that stores a negotiated discount, you create a field with the exact same API name on SBQQ__Subscription__c. When the quote is contracted, CPQ automatically copies the value to the subscription. On renewal, CPQ copies the value back to the renewal quote line. This avoids the need for custom Apex or Flow to synchronise custom data through the quote-to-contract-to-renewal lifecycle. Twin Fields work for all data types. For fields that should map between Quote and Order Item, CPQ uses a similar mechanism driven by matching API names across SBQQ__QuoteLine__c and OrderItem.