When a renewal Opportunity is created using the Swantide workflow, automatically create renewals, the ARR from the parent Opportunity will be brought into the renewal - we typically call this field Parent Opportunity ARR or Up for Renewal ARR.

Then, when the Renewal's ARR is calculated, we will use the delta of the Renewal ARR and the Parent Opportunity ARR to find the Net New ARR. If this is a renewal with expansion, the Net new ARR will be a positive number; whereas if this is a renewal with partial contract churn, it will be negative; alternatively, if there is no change in ARR, the number will be zero.
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With the Net New ARR field you are able to pull reports on Opportunities that are Closed/Won to track the effectiveness of renewals, how often they upsell/downsell, or remain the same.

Note: Expansion can also be calculated through a separate Opportunity with Type "Expansion." We recommend creating a separate Expansion Opportunity for mid-cycle upsells since they typically represent a separate deal cycle.