Salesforce Formula (Chapter 2)

IF(
  RecordType.Name = 'Renewal',
  Renewal_Of__r.ARR__c,
  NULL
)

Explanation:

This formula captures the ARR from the previous Opportunity, allowing us to track the value that was up for renewal. If the Opportunity is not a renewal, the field remains empty.

Salesforce Formula (Chapter 3)

IF(
	RecordType.Name = 'Renewal',
	Renewal_Of__r.ARR__c + BLANKVALUE(Renewal_Of__r.Total_Amendment_ARR__c, 0),
	NULL
)

The ARR Up for Renewal field sums the base ARR and any amendments for a renewal Opportunity, ensuring a complete view of the total revenue up for renewal.