Which usage summary field can be used as an external IDto simplify usage uploads afteramendments?

 The matching ID field is used to link usage records to usage summaries based on a common identifier. This field is especially useful when there are amendments to the order products, as it allows usage records to be uploaded to the correct usagesummary without having to change the order product ID or the usage summary ID. The matching ID field can be used as an external ID to simplify usage uploads after amendments, as it can be populated by a workflow rule or a formula based on certain criteria. 12 References:

Usage Summary and Usage Fields - Salesforce

Differences between the ‘External ID’ field and the ‘Unique ID’ field setting - Salesforce