There is no separate usage report for Reserved Instance (RI).
Option 1: RI owner/Account Admin can download the RI Invoice from the Azure portal.
Option 2: If ACM is enabled for RBAC roles and you have the right RBAC role, RI charges would be shown on ACM.
Reference link - https://learn.microsoft.com/en-us/azure/cost-management-billing/reservations/view-reservations?toc=/azure/cost-management-billing/toc.json
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